a tricky question because when it comes down to it, very little characters confirm that they're ethnically japanese (i think it's just one of those "default" situations), but here are the confirmed characters!

• adonis (half of swana descent)
• aira (quarter french)
• ibuki (ryukyuan, or okinawan)
• hiiro (indigenous, implied ainu)
• rinne (indigenous, implied ainu)
• hitsugi (mixed indigenous, half australian indigenous, half implied ryukyuan, or okinawan)
• negi (mixed indigenous, half australian indigenous, half implied ryukyuan, or okinawan)

priest is also implied to be indigenous (implied ryukyuan, or okinawan) and iirc gatekeeper isn't japanese either but idk much about that guy KJSDFS

worth noting that it's also randomly implied there's a possibility mayoi and kanata have ethnically indigenous roots based on histugi's story in Grand Slam - Blue Requiem 6? but we don't know about that it's not confirmed

edit: also worth noting that for rei & ritsu, they may have ancestral roots in romania! they call it the "holy land of their family" and visit it regularly!

hopefully i didnt forget anyone!