I read all the books again this month to have everything fresh in my mind for LCR and a few things stuck out to me. Spoiler for LCR in the middle, I'll warn again before I get there.
My #1 suspect (besides my conspiracy theory that it's Cody from the restaurant) is that Volto is Jay. However, after my re-read I picked up that Sang describes Jay as a "brick wall" -- or something to that effect, I can't remember the exact wording. I believe this was in First Days. Then in THPOS when Luke tackles Volto, Luke describes him as sort of regular, nothing stood out and that maybe it even could have been a tall girl with small boobs. I think Jay's stature would be easy to describe. If Volto was Jay, Luke's description would have been clearer (ex: he was big, muscular etc.) So he's off my list.
My next suspect was Wil. He was always in the story just enough to forget about, but always in the background enough to remember him when he interacted with Sang. The fact that he has hacking skills, can be in and out of school whenever and is on his own can be proof it would be easy for him to be Volto. Then I read LCR
Wil has a new job in this book and the timing just doesn't add up. He couldn't have been working and Volto at the same time. Plus the way he was protective of Sang in this scene was different than the way Volto is "protective" (at least in my opinion). I won't say anymore about it to not spoil the book, but once you read LCR it will make sense. So I crossed Wil off my list.
That leaves Karen. I'm still not convinced it's her, but I would need to read the books again to try and pick up Karen clues since she was last on my list I wasn't really paying attention to her. Things that make me wonder...yes she's an athletic girl, but can she really over power the guys?? I know Luke was sick that one time and therefore weaker. Her conversation with Sang at the football game about dating all the guys and kissing etc. doesn't feel like something Volto would talk to Sang about if Karen is Volto. Unless Karen as Volto was just fishing for info on the guys. Also did we see Karen at school after the guys crashed into Volto with the car in DoD?
Sorry for all the rambling lol!! I just wish Volto would stop with the games! If Volto really wants to help and wants Sang out of danger he needs to tell them where the money is that Hendricks is stealing and how he is doing it!! The team's job would be over and Sang would be out of danger!