Complete all eight tournaments, then replay any of the tournaments as the indicated character to unlock their concept art.

Adjo's ending: Replay a tournament as Adjo.
Dragov's ending: Replay a tournament as Dragov.
Finn's ending: Replay a tournament as Finn.
Killen's ending: Replay a tournament as Killen.
Kuro's ending: Replay a tournament as Kuro.
Nemo's ending: Replay a tournament as Nemo.
Renwick's ending: Replay a tournament as Renwick.
Seager's ending: Replay a tournament as Seager.