Yeah same boat, I had actually unlocked him played a few duels with him. Then I came back and he was unplayable. I also noticed a skill I had unlocked on Kiba is now unusable in the sense I havent unlocked it even though I have. Might be something with their server issues / rollback they might have done. Not sure.
Have you guys tried reinstalling the game? Make sure to save your account first in the settings. If that doesn't work then contact the in game support.
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Uroucyon_GameA
Ethan