Complete all 8 Tournaments, then replay any of the Tournaments as your preferred character to unlock concept art of them.

Art	How to Unlock
Adjo Concept Art	After finishing all 8 Tournaments, complete any Tournament as Adjo.
Dragov Concept Art	After finishing all 8 Tournaments, complete any Tournament as Dragov.
Finn Concept Art	After finishing all 8 Tournaments, complete any Tournament as Finn.
Killen Concept Art	After finishing all 8 Tournaments, complete any Tournament as Killen.
Kuro Concept Art	After finishing all 8 Tournaments, complete any Tournament as Kuro.
Nemo Concept Art	After finishing all 8 Tournaments, complete any Tournament as Nemo.
Renwick Concept Art	After finishing all 8 Tournaments, complete any Tournament as Renwick.
Seager Concept Art	After finishing all 8 Tournaments, complete any Tournament as Seager.