Each of your allies fall into the following classes.

Class	How to unlock
Archer (Yellow	Can fire arrows from any distance at enemies in the same row.
Cleric (Green	No offensive ability, but can heal nearby allies.
Knight (Blue	Good defensive abilities, average offense. Their shields allow them to take reduced damage from ranged magic and arrows.
Soldier (Red	Good offensive abilities, average defense, but vulnerable to ranged attacks.
Wizard (Purple	Shoots ranged magic from any distance at enemies in the same row.