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

Adjo concept art: Replay a tournament as Adjo.
Dragov concept art: Replay a tournament as Dragov.
Finn concept art: Replay a tournament as Finn.
Killen concept art: Replay a tournament as Killen.
Kuro concept art: Replay a tournament as Kuro.
Nemo concept art: Replay a tournament as Nemo.
Renwick concept art: Replay a tournament as Renwick.
Seager concept art: Replay a tournament as Seager.