What are you guys saying? He's not Xerx, He's just pretending to be Xerx. Apparently Xerx did an expedition and by the time he arrived at the Clover kingdom and the guy who was pretending to be "Xerx", defended an old woman that was in distress because of the real Xerx and he defeated the real Xerx and took the robe and went to find some more Magic knights to humiliate.
and the wizard king is clearly acting that he did not notice, you can notice it when the wizard king laughed and said "Looks like he gave himself quite the makeover after that long expedition of his" and then his assistant said "This is no laughing matter, Sir!" and then look at the Wizard king's face. You can clearly notice it.