You can unlock the character's model in the Archive menu by finishing arcade with that character.

Model	How to unlock
Chrono's Model	Finish arcade with Chrono.
Fate's Model	Finish arcade with Fate.
Hayate's Model	Finish arcade with Hayate.
Material-D's Model	Finish arcade with Material-D.
Material-L's Model	Finish arcade with Material-L.
Material-S's Model	Finish arcade with Material-S.
Nanoha's Model	Finish arcade with Nanoha.
Reinforce's Model	Finish arcade with Reinforce.
Shamal's Model	Finish arcade with Shamal.
Signum's Model	Finish arcade with Signum.
Unison Reinforce's Model	Finish arcade with all characters.
Vita's Model	Finish arcade with Vita.
Zafira's Model	Finish arcade with Zafira.