I feel really creative this week so… If Killua’s an assassin then he should have more than one look and be able to act to get the job done, right? So think about this, along with the body manipulation technique where he makes his fingers sharp, he can also change what he looks like entirely. He found out by accident, when he was still learning how to change his fingers, but he accidentally changed his hair color. So he experimented with it, and found out he could also change his eye color and his skin color. It took a lot of practice, but he finally did all three at once. But when he met Gon, he completely forgot about it. So when Gon leaves, he wishes for him, Alluka, and Nanika to be somewhere where Illumi won’t find them, they end up in the Mha universe and Killua remembers about his full body manipulation technique, as he calls it, but when he did it again, something went wrong. Maybe it was because he had nen, or he was a little unstable at the time, but he accidentally awakened a split personality, and he could interact with it, but there was also a Gon in his head too. Maybe this wasn’t all that bad, at least he still knew how to use Nen.