Realms
There are 3 types of realm to choose from: Man, Gods, and Demon.
The realm you pick can give bonus to your cards' attack/defense in battle if they match your realm.

Evolve - You can evolve cards by putting 2 of the same cards together ,examples are:

Soldier + Soldier = Soldier+
Next you can do is: (Soldier+) + Soldier= Soldier++ and the third time will bring them to their final form. Every time you evolve the evolved card will keep 5% of their stats, it would be 10% carried over instead. It also upgrades their rarity from Normal to High Normal, High Normal to Rare etc.

Here are some hard number info

8
8-14 (tier 1,2,3): 100%
8-6 (tier 2,3): 99.70%
8-2 (tier 3): 97.87%
8-0: 87.13%

7
7-12 (tier 1,2,3): 99.81%
7-6 (tier 2,3): 99.59%
7-2 (tier 3): 97.85%

6
6-10 (tier 1,2,3): 99.63%
6-6 (tier 2,3): 99.48%
6-2 (tier 3): 97.81%

5
5-8 (tier 1,2,3): 98.50%
5-6 (tier 2,3): 98.43%
5-2 (tier 3): 96.85%

4
4-6 (tier 1,2,3): 96.63%
4-4 (tier 2,3): 96.55%
4-2 (tier 3): 95.73%
4-0: 86.63%

Enhance - Cards can be be leveled up by sacrificing any card that's not a dupe of the enchancee. How much xp the card gets depends on the enhancer card. Max level you can enhance cards are:

Normal 20
High Normal 30
Rare 40
High Rare 50
Super Rare 60
SS Rare 70
Legendary 80


Stats:You get 3 points to spend when you level up from questing and 5 points when adding a fellow:

Stamina - pool that's used to do quests, usually you spend sta 3 times before ending a round and get your card, money or treasure.
Atk - Are used for battles to beat other people for their money, and even mugging them of their treasure if you target it.
Def - For defending against other muggers? Couldn't figure out how to use this stat proactively but the more you have, the better your wall against possible robbers of your loot.

Leader card - choosing leader card will make it your account's avatar, it also changes the attack animations in quests a little bit. Picking the leader doesn't matter much except for boss fights which uses your lead's Atk stat.

If you try it out and get done with tutorial, you can add a referral code (mine is qwz81510 ) in the end and you get a decent rare card to start with. You can also fellow add me(Cloudberryjam)and my guy can help in boss fights, trade cards with, and maybe even progressing in quests(but i haven't figured it out yet how though).
I've been hitting this up. Make sure to u a referral code like Mungbutter's when you start - you get a ton of cash and a really good rare (Angelic Knight).

The game is pretty fun; there's always something to do at first. You get a ton of cards all the time (every 3 times you spend stamina on quests, and also a ton of other means), and you just feed trash cards you get to your other cards to boost them up.

It's quite like Mafia Wars but with Yugioh cards you collect and power up like Pokemon. Plus the cannibalism thing.