There are many hidden characters to recruit, and even the non-hidden ones can be difficult to track down! Here's a list of who each protagonist can recruit, and how to get them!

I will be editing and updating this guide as more information about the newly added content is discovered, as well as formatting it for quicker reference and readability.

Just be aware, spoilers below! For a spoiler-free list of who can recruit who, check the Quick-Reference Table! :)

1) Annie
2) Asellus
3) BJ&K
4) Blue
5) Captain Hamilton
6) Cotton
7) Doll
8) Dr. Nusakan
9) Emelia
10) Engineer Car
11) Fei-On
12) Fuse
13) Gen
14) Ildon
15) Kylin
16) Leonard
17) Liza
18) Lute
19) Mei-Ling
20) Mesarthim
21) PzkmW
22) Rabbit
23) Red / Alkaiser
24) Rei
25) Riki
26) Roufas
27) Rouge
28) Sei
29) Silence
30) Slime
31) Suzaku
32) T260G
33) Time Lord
34) Thunder
35) Turnip
36) White Rose
37) ZEKE
38) Zozma

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1) Annie
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Gender: Female
Talents: Swords / Martial Arts
Race: Human
Joins: All

Lute, T260G - Once you've managed to reach the side quests, sign up for the Rune Quest in Devin. Get at least 1 Rune and then return to Koorong. On the top level, there is a woman standing in front of the restaurant with orange hair and green clothes. Talk to her, she'll ask you to go to Despair to get the next Rune. Accept her offer and she'll join you.

Asellus - Mostly the same, though it's reported Annie won't join unless the Freedom rune is the only rune left to obtain.

Blue - Sign up for the Rune Quest in Devin. Unlike other heroes, Blue does not need to acquire a rune first. Just go to Koorong's top level, and talk to the girl with orange hair and green clothes. She'll ask you to go to Despair and get a rune. Accept her offer and she'll join you.

Emelia - Annie joins your team very early, and plays an intricate role.

Red - Once you've managed to reach the side quests, visit Koorong. On the top level, there is a woman standing in front of the restaurant with orange hair and green clothes. She'll offer Red information on BlackX over dinner. After a scene inside the restaurant, she can be recruited by speaking to her again outside and telling her that you're ready for her to join. This is safe to do, as it does not force you to go to the BlackX base right away.

Riki - When you are searching for the Hermit ring, talk to Annie and she'll join your quest.

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2) Asellus
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Gender: Female
Talents: Swords / Magic / Mystic Weapons
Race: Half Mystic / Half Human
Joins: Emelia, Red (Temporary)

Emelia - On your fourth mission, when you go to Trinity, she will be right outside the room you first start in. Go and talk to her. You'll be attacked, but the monster is fairly easy.

Red - Early in Red's quest, the Cygnus will be attacked by a pirate ship. Inside one of the passenger's rooms, Asellus and White Rose will join you until the end of the mission, then depart for good.

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3) BJ&K
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Gender: N/A
Talents: Health Recovery (Non-Mechs), Heavy Weapons (ex. Hyper Blaster)
Race: Mech
Joins: Red

BJ&K is the ship's doctor aboard the Cygnus, and will heal you any time you visit his room, right next door to the room where Red collects his pay. Once the ship is hijacked, go back to his room to find a pirate inside instead. Defeat the monster, and then leave and re-enter the room. Talk to BJ&K and he'll join you.

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4) Blue
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Gender: Male
Talents: Magic
Race: Human
Joins: None

Blue can never be recruited by any other hero. (Unknown if Fuse is an exception yet)

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5) Captain Hamilton
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Gender: Female
Talents: Guns or Martial Arts
Race: Human
Joins: Lute

Go to Owmi. In the restaurant, talk to Captain Hamilton. She will leave and go to Nelson. Follow her there and talk to her. She will join you.

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6) Cotton
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Gender: N/A
Talents: Monster Skills
Race: Monster
Joins: All

In Shrike, go to the Bio Research Lab and make your way to the top floor. Open a secret door in the bookshelf by the door on the left. Go up and fight the scientists. Once you win, Cotton will join
you. There are bonus dialogues if you have Fuse and/or Doll in the party when you rescue Cotton.

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7) Doll
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Gender: Female
Talents: Guns / Magic
Race: Human
Joins: Red

Red - Inside the shingrow stating, you'll see a woman with purple clothes and white hair. Talk to her and agree to helping her find her brother and she'll join your quest.

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8) Dr. Nusakan
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Gender: Male
Talents: Mystic Weapons
Race: Mystic
Joins: Asellus (new content), Blue, Riki

Asellus - After escaping the Dark Labyrinth, visit Koorong and interact with Zozma outside the port, who will mention a high-class mystic that lives in the city. After the dialogue you can visit Dr. Nusakan's office and speak to him. The good doctor will be quite interested in Asellus' Half-Mystic biology, and will join you.

Blue - Once you are on the Rune quest. Go down the pathway in the bottom of Koorong until you see a door with a rune sign over it. Go into the building and ask the head if your being served. Then you can go in and ask Dr. Nusakan about the runes. He'll join you.

Riki - When you are in the billionaire's house in Yorkland, fight the demon. Mei-Ling will then send your team back to Koorong. Go down to Dr. Nusakan's office. Talk to him, explain your problem, and he will join you.

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9) Emelia
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Gender: Female
Talents: Gun (Then Martial Arts)
Race: Human
Joins: All

Visit Devin and start the Arcane Quest. Go to Baccarat and talk to the woman in a blue bunny suit about the Gold Card. Follow a gnome to the top floor where you'll just miss him going down the elevator. Head down to the parking lot and go down the sewer. Emelia will ask to join you.

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10) Engineer Car
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Gender: N/A
Talents: Repairing Mechs, Heavy Weapons (ex. Hyper Blasters)
Race: Mech
Joins: Blue, Lute, Red, Riki, T260G

You must first have at least one Mech already in your party (T260G's the obvious choice). Then, go to Shrike and enter Nakajima Robotics. By the staircase, there is a mech. Talk to him and he'll join you.

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11) Fei-On
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Gender: Male
Talents: Martial Arts
Race: Human
Joins: All but Blue

Riki - After defeating Platyhooks in Nomad's hideout and progressing further, Fei-on will tell Riki that the way forward is dangerous. Tell him you don't want to proceed, then talk to him and he'll join.

Others - Start the Rune quest. After getting at least one Rune, attempt to travel away from Koorong to no avail - you'll be swallowed by Tanzer. Once inside, follow Fei-On to the encampment inside of Tanzer and go into his room. Agree to help him and then he'll join your quest.

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12) Fuse
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Gender: Male
Talents: Swords
Race: Human
Joins: All

Red - You get Fuse temporarily on the Cygnus early on in Red's quest. After leaving the Cygnus at Yorkland, visit Manhattan to find Fuse inside the restaurant on the second floor. Simply speak to him and he will rejoin permanently.

Others - Go to Devin and sign up for the Arcane Quest. Head to IRPO and talk to Fuse. He will take you to Mt. Mosperiburg. Climb the mountain and defeat Suzaku. When you return, Fuse will join your team, and you'll have the Shield Card too. ( Unknown: Possibly subject to change with Remastered content enabled! )

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13) Gen
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Gender: Male
Talents: Swords / Martial Arts
Race: Human
Joins: All

Asellus, Blue, Emelia, Lute, Red - Sign up for the Arcane Magic Quest. Head to Koorong and talk to the skeleton on the lower level. The skeleton will talk about Gen and mention he may be from Wakatu. Head to Scrap and talk to Gen in the pub. He'll join your team. (For an amusing mini-scene, visit the skeleton again with Gen in the party!)

Riki - Talk to Gen in the pub in Scrap. Try to leave and he'll join your quest.

T260G - Gen plays an intricate role and will join you very early in this quest.

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14) Ildon
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Gender: Male
Talents: Mystic Abilities
Race: Mystic
Joins: Asellus

In the beginning of this quest, talk to Orlouge and Ildon will join you briefly. He will eventually leave, but you can get him back after defeating the Lion Princess.

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15) Kylin
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Gender: N/A
Talents: Monster Skills & Space Magic
Race: Monster
Joins: All but Blue

To get Kylin, you have to complete either the Rune or Arcane quest and the Light or Shadow magic quest (one from each pairing). Once you've done that, go to Devin and talk to Rei. She'll warp you to Kylin's Paradise. Talk to Kylin, take his challenge and defeat him. Talk to him afterwards, and he'll join your quest.
⠀⠀⠀⠀⠀⠀⠀___________
______/ Rei AND Kylin?⠀\______________________________________
| Unless you move fast, you can only get one or the other. If you want both in⠀⠀⠀⠀⠀|
| your party, you'll need recruit Kylin before the Dark Labyrinth event occurs. ⠀⠀⠀⠀⠀|
| This means unlocking the gift for Light/Shadow and the gift for Rune or Arcane, ⠀⠀|
| then visiting Kylin's Realm through Rei fairly quickly. Good luck!⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀|
|_______________________________________________________|

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16) Leonard
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Gender: N/A
Talents: Mech Skills, Heavy Weapons (ex. Hyper Blasters)
Race: Mech
Joins: T260G

Go to the Shingrow Ruins, and complete that event. Then return to Manhattan (Leonard's Lab) and talk to him. He'll join you.

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17) Liza
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Gender: Female
Talents: Martial Arts
Race: Human
Joins: Blue, Emelia, Red

Blue - Sign up for the Rune Quest. Go into Shrike's "Mu's Tomb" and get Roufas. Head back to Koorong and the restaurant where Liza will gladly join you.
⠀⠀ ________
__/ Runic Duo \____________________________________
| If you are going to get Liza, first get Roufas. Once Roufas joins⠀⠀⠀⠀⠀|
| the team, right away, get out of there and get Liza. If you get the⠀⠀ ⠀|
| Rune first, you cannot get Liza. As one of the game's best Martial⠀ ⠀|
| Artists, Liza can get the Dream Super Combo very quickly.⠀⠀⠀⠀⠀⠀⠀⠀|
|______________________________________________|

Emelia - Liza plays an intricate role in this quest. She'll join you very early along with Annie.

Red - Talk to Annie and she'll offer to take you to Shuzer's base. Once you've recruited Annie, simply head back into the restaurant and speak to Roufas. Liza will come out from the back. After their conversation, Roufas decides the three of them need to split into teams. Annie declares she's
with Red, so Roufas says to pick one more - Liza or himself. Take Liza because you can still get Roufas afterwards through his normal recruitment method at Mu's Tomb. (Note: As Red, Liza must be recruited BEFORE Roufas if you want them both!)

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18) Lute
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Gender: Male
Talents: Any
Race: Human
Joins: All

Go into the Scrap pub, tell him your story, and he'll join your quest.

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19) Mei-Ling
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Gender: Female
Talents: Guns
Race: Human
Joins: Blue, Emelia, Lute, Riki, T260G

Blue, Emelia, Lute, T260G - Go into the Scrap pub, tell her your story, and she'll join your quest.

Riki - Mei-Ling plays an intricate role in Riki's quest. She'll join you very early on.

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20) Mesarthim
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Gender: Female
Talents: Mystic Skills
Race: Mystic
Joins: All but T260G

Note: Mesarthim will not join you if you have Silence, but not the other way around - you can recruit him after you already have her with no problems.

Asellus - Mesarthim joins you early in this quest. She does leave again, so to get her back, simply go back to where you released her inside the Lord's Manor after completing the Dark Labyrinth and she'll join your quest again. This will require defeating the Kraken blocking the path both on the way in and out, however.

Others - Go to the Lord's Manor in Owmi and go to the bottom floor. Talk to Mesarthim and she'll join you IF you have another Mystic in your team.

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21) PzkmW
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Gender: N/A
Talents: Mech Skills, Heavy Weapons (ex. Hyper Blaster)
Race: Mech
Joins: T260G

Once you get Leonard, talk to the owner at the gun shop in Koorong's backstreet (where you buy Hyper Blasters) and he'll join you.

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22) Rabbit
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Gender: N/A
Talents: ECM / Bit System, Heavy Weapons (ex. Hyper Blaster)
Race: Mech
Joins: Red

Only when playing as Red, you can find Rabbit inside the garden in Kyo. Talk to him and he'll join you.

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23) Red / Alkaiser
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Gender: Male
Talents: Swords (or Martial Arts)
Race: Human
Joins: None

Alkaiser can be activated only when all Humans in the party are dead or status-inflicted. Be warned: while Alkaiser is a powerful boost, using Alkaiser's form means Red will not level up in any way at the end of that battle!

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24) Rei
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Gender: Female
Talents: Mystic Skills
Race: Mystic
Joins: Asellus

First, talk to White Rose without Ildon present while in Rei's former room in the beginning of the game. Later on, when you beat the Dark Labyrinth, talk to the girl who warped you to Kylin. She'll reveal herself as Rei and join your team.

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25) Riki
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Gender: N/A
Talents: Monster Skills
Race: Monster
Joins: Blue, Emelia, Lute, T260G

Talk to Riki in the Scrap pub and he'll join you, no questions asked.

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26) Roufas
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Gender: Male
Talents: Katanas / Guns / Magic
Race: Human
Joins: Blue, Emelia, Lute, Red

Blue / Lute - Sign up for the Rune quest in Devin and then talk to him in Mu's Tomb.

Emelia - Roufas plays an intricate role in Emelia's quest. He will join automatically, though you can get him earlier on, without needing to visit Devin first, by simply visiting him in Mu's Tomb after your first 'mission' with Gradius,

Red - Talk to him in the Cygnus and he'll join you briefly. Once you can, talk to Annie and recruit her to go to the BlackX base. Once you've got Annie, head back and you'll get a choice of having Roufas or Liza join. You can get Roufas here, but it's recommended you take Liza and then go to Mu's Tomb. There he'll join you if you're on the Rune quest, and you'll have both Roufas and Liza.

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27) Rouge
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Gender: Male
Talents: Magic
Race: Human
Joins: Asellus, Blue*, Emelia, Lute, Red

Blue - When the time comes to defeat Rouge, lose the battle.

Others - Go to Luminous or Devin and talk to him in the ports. He'll join you from there.

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28) Sei
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Gender: N/A
Talents: Monster Skills
Race: Monster
Joins: All

Go into Sei's Tomb and collect his three treasures (a sword, shield and accessory) from within. Once you have them, return to the second room and place them on the now glowing pedestals. The door will open and inside you will be able to fight King Sei. If you win, you can take his sword, the Kusanagi, or have him join. If you don't want Sei or the Kusanagi, it can often be better to keep his three treasures and leave without placing them on the pedestals!

Note that for Riki's quest, recruiting Sei is mandatory. No treasures for you, green boy!

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29) Silence
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Gender: Male
Talents: Mystic Skills
Race: Mystic
Joins: Asellus, Blue, Emelia, Lute, Red

Go into the Shadow Magic Dungeon (Omble) and talk to Silence's shadow. Once you escape the dungeon, talk to Silence to the side of the entrance and he'll join you. This can only be done if your protagonist does not already have the gift for Light Magic!

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30) Slime
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Gender: N/A
Talents: Monster Skill
Race: Monster
Joins: All but Riki

Sign up for the Rune quest and obtain at least one rune. Then, try to travel away from Koorong and you'll be swallowed by Tanzer. Go through Tanzer and get the Rune inside of him after defeating the slimes. When you get out, Slime will have joined you.

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31) Suzaku
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Gender: N/A
Talents: Monster Skills
Race: Monster
Joins: All

Suzaku is a powerful monster character which starts with some unique abilities (be sure not to unlearn them!), but when you travel to Mt. Mosperiberg from IRPO in search of the Shield Card, you'll find it's been trapped and frozen in a block of ice by the evil snowman, Jotnar. Defeating Jotnar is the only way to free Suzaku, so to make him appear you must first defeat the fairy at the bottom of the trail on the first screen. This will make a snowman appear on the screen where you find three boars. However, it will only be Jotnar if the snowman has appeared in the bottom center area of that screen. If it's in another spot, go back and kill the fairy on the first screen again. Repeat until the evil snowman himself shows up, then defeat him (Be warned: this is a difficult battle! Bring strong defenses vs Ice and Water attacks!), and return to the cave where Suzaku will now be free to join you.

Note: A hot tip has it that if you want Suzaku as Red, you must resist the opportunity to pick up Fuse for free immediately at the mall. Start the Tarot quest first and only then recruit Fuse at IRPO as you go for the Shield card with him. Obviously this means you'll have to go for this card first if you want Fuse to know the gift for Arcane magic, so plan accordingly!

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32) T260G
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Gender: N/A
Talents: Chassis Swap, Mech Skills, Heavy Weapons (Ex. Hyper Blaster)
Race: Mech
Joins: Blue, Lute, Riki

Go to the Scrap Pub and talk to him. He'll join you. T260G can swap his chassis by visiting Nakajima Robotics in Shrike, but only if he is the protagonist.

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33) Time Lord
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Gender: Male
Talents: Mystic Skills & Time Magic
Race: Mystic
Joins: Asellus, Emelia, Lute, Red

Get the gift of Light and/or Shadow Magic, plus Rune and/or Arcane magic and then go to Mosperiburg. Virgil will teleport you to the Time Lord's area where you'll find that time has stopped. Investigate the hourglass' spilled sand and then return to Virgil. Virgil will now teleport you to Facinaturu where you can acquire the Sand Vessel from Gozarus, at the permanent cost of one maximum Life Point from your protagonist. To leave Fascinaturu, talk to the pilot hanging out in Rootville's inn, go through the cave again and get on the plane. Now return to Virgil who will warp you back to the Time Lord's realm. Gather sand from the bottom, then jump on top of the hourglass and the vessel will activate. Time will flow again, and you can open the drawbridge. Talk to the Time Lord and he'll ask why you restarted his clock - say it was to get his help, and he'll join you.

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34) Thunder
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Gender: N/A
Talents: Monster Skills
Race: Monster
Joins: All

Lute - Go back to Yorkland and talk to the big orange ogre. It's your 'little' bro (and newest team member!)

Others - Recruit Lute first at the pub in Scrap, then as above.

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35) Turnip
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Gender: N/A
Talents: Monster Skills
Race: Monster
Joins: Asellus

In the Dark Labyrinth, talk to the lone monster. After you exit, he'll join you.

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36) White Rose
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Gender: Female
Talents: Mystic Skills, Magic
Race: Mystic
Joins: Asellus, Emelia, Red (Temporary)

Asellus - White Rose plays an intricate part in Asellus' quest. She will join you early on.

Emelia - On your fourth mission in Trinity, talk to Asellus. Both Asellus and White Rose will join your team.

Red - Early in the quest, when the Cygnus is being attacked, White Rose and Asellus will join you if you can find them in their cabin on the 2nd floor. However at the end of the mission, they both depart for good.

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37) ZEKE
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Gender: N/A
Talents: Mech Skills, Heavy Weapons (ex. Hyper Blasters)
Race: Mech
Joins: T260G

Go to Sei's Tomb and capture the robotic mouse. Bring it back to Nakajima Robotics and they'll let ZEKE join you.

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38) Zozma
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Gender: Male
Talents: Mystic Skills
Race: Mystic
Joins: Asellus, Emelia

Asellus - After you beat the Dark Labyrinth, Zozma will join your quest.

Emelia - Once you get White Rose and Asellus, search the Trinity Base and you'll find a solider. It's really Zozma! Talk to him and he'll join your quest.

This guide is directly is based on the work originally done by Greg Protomanni over at GameFAQs, which you can find here! - https://gamefaqs.gamespot.com/ps/198537-saga-frontier/faqs/7108

The information within has been edited heavily from the original version due to corrections, formatting, streamlining and new content by myself.

Unfortunately, Greg's provided email address is no longer valid, but I'd like to give a Big Thank-You to him for all his hard work! :D Greg, please contact me if you have any questions or concerns! ♥

Special Thanks to my commenters for helping correct things as we go:
Auspice - Riki's recruitment info for Fei-On
Lucifer Nevermind - Corrections to BJ&K and Suzaku
Straw & Pixel Ripper - Helpful updates on recruitment for Fuse
Seijun - Finding a method to allow Asellus to recruit both Rei and Kylin!
Whooda - Giving me the heads-up on Red being unable to recruit Suzaku
MasterZoen - "Gen can punch good!" Woo! :D
renegadephoenix6 - Pointed out that Sei always joins Riki, confirmed by MasterZoen. :3
Jeff_The_Glitched - Pointed out the method to recruit Suzaku with Red