User blog comment:Nerdyanimegirl225/The Worst. The Best, and why?/@comment-4829046-20140525053858/@comment-4829046-20140525114113

I don't recall it being stated that the Zero personality sealed itself up out of its own volition. But technically since we never really got any info on when and how he was first sealed plus since they are the same person, I guess that applies. I think more on the grounds that one time he only had one personality but he preferred going for fame, power and glory and discovering and creating new things but his urge to destroy everything contradicted it so he sealed those emotions and due to that, they became a completely independent personality.