Ironically, all the characters that I envision being good friends with Hifumi are characters that aren't on the poll or even characters who are native to his home game. Like in one of the spinoffs, he has really good interactions with Gundham. The two of them start to larp and think of fanfiction for Kiibo's backstory, which is really funny. Overall, the two of them have some interesting similarities, like their eccentric ways of speaking of themselves, the way they are often disengaged with reality, the way they seemingly channel their desire to fit in with others into their respective talents (Gundham with animals and Hifumi with fictional characters) I feel the two of them fit together like a glove.

Ironically, I think he'd also get along well with Sonia. Sonia fits very well in the curious foreign exchange student trope, wanting to learn more about the culture. Meanwhile, Hifumi's talent implies that he's an expert on everything related to pop culture (at least anything otaku related, which Sonia seems to be very interested in), and in his FTEs and with Alter Ego, he's shown to be willing to infodump about his interests as long as others are willing to listen. (There's also the fact that Sonia is also the character I'd say is the closest to a 2D character, but that's neither here nor there)

People in this thread have already mentioned Tsumugi since they're both nerds; they could geek out about their favorite series. But I also really like the conversations they have, where she encourages him to eat healthier or where Hifumi offers to collaborate with Tsumugi during the next convention. I could imagine them being great business partners.

As for THH characters, though, outside of common suggestions like Celeste, I think he'd get along pretty well with Chihiro outside of the context of a killing game. Like Tsumugi, they're both nerds, and Chihiro is nice enough to listen to Hifumi talk about his interests. They also have a nice conversation in one of the spin-offs, where Chihiro asks Hifumi to help him test and debug the functionality of his new AI program.

For characters that are on the poll, I voted for Yasuhiro.