Get a B or Better on Armin's story mode to unlock. An A or A+ is not required by it's highly possible to gain.

Unlockable	How to unlock
Armin set	Mission: Mercy.
Christa set	Mission: Female Titan.
Cis hair and face	Mission: 57th Exterior Scouting Mission.
Franz hair and face	Mission: A Dim Light Amid Despair.
Hanna hair and face	Mission: A Dim Light Amid Despair.
Jean set	Mission: Defend to the Death.
Mitabi hair and face	Mission: Counterattack.
Ness hair and face	Mission: 57th Exterior Scouting Mission.
Reiner set	Mission: Female Titan.