So, uh... what was the deal with the Ultimate Imposter? Why did he pretend to be Byakuya, why did he even know who Byakuya is, and why did he he pretend to be fat? Why did he take charge of everything so quickly and why was he so dedicated to everyone's safety? Why did he seem to know what was going on the entire time?
My take is that the imposter was going to impersonate someone. That's what he does. That's who he is. And he just so happens to be fat. But he acted exactly like Byakuya would in those circumstances. It is a little odd that he has some semblance of memories of what took place prior, when nobody else appears to, but we also see at the end that the "other" version of Hajime appears to Nagito as they're arriving on the island, so when where and how memories get altered isn't completely clear.
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Video Game Music Contest 14 champion: Idola Phantasy Star Saga - Endeavour