阿J

One of my favorite things about Volo's reveal is just how much the game builds up us having a bond with him beforehand.

He is kind to us and supports us the entire game, even initially being the only person there for us in our darkest hour, when Kamado has us exiled, and is keeping the Diamond and Pearl clans from helping us under threats of war. (And if Laventon, the rival, and Cyllene openly help us, they will suffer the same fate we do, so they can't help either.)

And while a player may have predicted he was evil prior to your meeting at the Giratina statue (I personally didn't see it coming), the player character themselves in the context of story has no real reason to distrust him beforehand. So, I could only imagine how shocking that betrayal was, to learn he was responsible for the main conflict, that he was only using us to get to Arceus, and that he was willing to murder us (and erase everyone and everything else) all with a smile on his face.

It makes him so much creepier than someone like Ghetsis, who in his person-to-person interactions with us, is clearly an asshole and not to be trusted even before he reveals the truth to N.