Unlock KFJ:
Beat KFJ (he is member of the Rogue Team) in Class B Story mode.
 
Unlock Boneyard:
Beat Boneyard (Team Phantom) in Class D Story mode to unlock him in arcade
mode.

Unlock Cinder:
Beat Cinder and her team in Class A Story mode to unlock her in arcade mode.

Unlock Shaman:
Beat Shaman and his team in Class B to unlock him in arcade mode.

Unlock Smoke:
Beat Smoke and his team in Class C to unlock him in arcade mode.

Unlock Hardcase:
Successfully finish arcade mode

Unlock Reina:
Successfully finish story mode