Here is a guide on how to unlock characters, char info.

Ratfolk Archerunnamed file 226

Golem Gotem – achievement
Unlocked by deafeating golem on the first map

The Divine

unnamed file 227

Feels good man – achievement
Unlocked by beating the Froghemoth ( last boss on the first map ?)

The Quickling

unnamed file 228

No trespassing – achievement
Unlocked by destroying portal in 30 seconds ( Second map )

Necromancer

unnamed file 229

Dawn of the dead – achievement
Unlocked by killing 1000 zombies ( Second map)


Thanks to Saiphe for his great guide, all credit to his effort. you can also read the original guide from Steam Community. enjoy the game.