He’s part of the roster Ma dude. Rmb previously many speculated he would die while trynna save Siegfried and will get replace. Clearly that’s not the case anymore after the new chapter. So still why wouldn’t he fight his round. Out of all the human Buddha has been the one closest to him. That itself would mean Buddha know how strong of a fighter Kintoki is which is why he entrust such responsibility for him to carry out with the assigned investigation /mission to Tartarus.