Was J-chan commission work paid by Mike?
If so Mike should had made a contract for the rights of the character stipulating Mike ownership and can argue so.
If it was a gift then you could technically say the artist owns it, but its still retarded to demand he stops using her when the character idea is 100% mike's and he can just get someone else to do a different model of J-chan.
Its a moot point regardless because Mike clearly doesn't want to use J-chan anymore, probably because the artist got b& for saying nigger and he would get canceled by his audience if they found out Mike was using the art of someone who said a no-no word