[It's definitely big Tragic Backstory Territory, that's for sure. Naoki seems to be waffling over it for a long moment before he finally begins to explain:]

Well... my world ended and became a Vortex World, right? There were only a few humans left. I knew all of them. Some of 'em were friends I went to school with, or my teacher...

But, they kept going off on their own. Bad things kept happening to them, and I wasn't around to help. [His brow wrinkles.] I was never in time.

[Thanks to Yang he doesn't blame himself as much as he used to, but... he'll probably always have a sense of responsibility for how things turned out.]