Mettaton Neo - Kill regular Mettaton
Undyne the Undying - Kill regular Undyne
So Sorry - Shift your time and date to October 10 8PM and restart your game
Follower 1 - Keep Press "F" on River person's head until he told you to "Beware of Someone who speaks in Hands"
Follower 2 - Spawn the Ancient Artifact until you get it
Follower 3 - Use the "Phone" and activate it until you get it
REDACTED - kill Every Follower
Mystery Man - you will unlock it randomly when restarting your game. For easier obtainement unsubscribe from every mod but this one.
Chara - Kill EVERY (And when i say Every i mean the ones that you can actually kill) Monster.
Goner Kid - Type "Goner" in the search bar.

There's another Unlockable, i don't know about, My theory that it is The Clam girl. I belive there's some conditions to turn it to the follower or something like that

YEAH! My theory confirmed! You need to spawn Clam girl over and over until a message pop up!