I agree, more established relationships (whether friendly or not, honestly) would be best. As for Eikichi and Haru, well [@Nani], I'm sure Takaya and Haru are actually friendly with each other for the reasons you mentioned! Takaya would most likely decline a lot of outings, but would agree to a few as its made clear Haru isn't letting up - at least until he starts getting more involved with everyone, in which case he finds it harder and harder to decline in the first place haha. And being on the student council, he would work together with Haru when they need his skills. 

This goes for Mikako and Eikichi too, at least while Eikichi was on the council in his first year. It's an easy starting point. 

[@Lemons] Mikako and Takaya are probably those that stick around the latest after council meetings. They might have eventually come to an agreement to swap the little duties after meetings (Mikako handles clean up and everything else on Takaya's cram school days, Takaya does it otherwise so Mikako can head to her other clubs or home to her family). I would say they are probably friendly, though Mikako and Haru would be closer to each other (and Eikichi) than Takaya? At first, at least.

[@OliveYou] And Eikichi, I think they would be cordial, if not a little friendly due to that familiarity in first year and him being friends with Mikako and Haru. But not very close. I would love if Eikichi knows or has an inkling that Takaya is a huge nerd and teases him with it. Perhaps some very mild black mailing to make sure Takaya agrees to Haru's latest invitations as not to bum Haru out hahaha. I think it also could be sort of interesting to play with Takaya's very traditional, plain look and facade versus Eikichi wanting to avoid being seen as plain and forgettable. Like, I feel like they wouldn't start off as very close friends but could become like that eventually - and then let Eikichi handle pushing Takaya to pursue his love interest haha.

[@ERode] And for others, with Junko becoming the manager of the judo club I expect that she and Takaya would start to see each other more often, even if only a little, especially if she needs to requisition funds for the club. Reading her quote about him, I can see him looking up to her as a senpai, seeing her as inspiring and intimidating in equal parts for many reasons, but he would certainly never presume to get friendly with the school's most beloved student. Although I think as things go on, the group gets tighter, and they start getting to know each other, I think it would be very cute if they became real friends. Assuming he follows the advice she keeps trying to give him.

[@Feyblue] Suzuka I actually feel would be the person Takaya might be least closest to, which is fine with me as it leaves her closest relationships to others. If you have any ideas though Fey, please let me know. It might be fun if they talk to each other on message boards about the latest battle shounens though, whether they know who the other is or not.

[@Vertigo] For Ryuu, perhaps as kids they were actually pretty close, but after Ryuu's trouble started and the separation, and now Ryuu turning back up as a (gasp) [i]delinquent[/i], they probably avoid each other. Or maybe its one sided and Takaya avoids him. So maybe some hurt feelings there, but ultimately they should become friends again. And if you want to go with Ryuu and Suzuka as cat friends but still want someone to know Ryuu through games, well Takaya is 'free' every Saturday for Ryuu's raid static haha. Could be a case where after discovering Ryuu is their crew's healer, he starts trying to mend their relationship through the game first before talking in person. Just a thought, of course.

Would be a similar case with Koharu in that his parents would definitely not approve of having a friend like that, so he'll have to get over it and just be friends with the people he likes.

[@Kensai] Which leaves Koharu! I think it would be nice if they were also close as kids, as they would have been similar as kids. And then to see her transformation would inspire some mixed feelings in Takaya. He's happy that she seems so happy, but also... not exactly jealous, the word is escaping me at the moment, but seeing how she embraces her true self as she grew up would make him feel some type of way. But yes, assuming they're in the same class (I just arbitrarily picked 2-2) he will be lending her all kinds of things haha. Eventually when he lets his cringe side come out more, he will lend her some manga too. In the mean time, he will be polite to her and probably classmates assume she is taking advantage of his nature before people learn they are just friends hahaha.

All that is more recent relationship stuff though. As for back then... I'm sure they would remember Takaya as a regular kid that hung out with them and eagerly played superheroes and such. After Suzuka, Koharu, and Ryuu left and both Takaya and Mikako's free time started evaporating, I imagine that some that remained behind went to different middle schools too. Maybe that was the case for Takaya, as even though he has lived in the city the whole time he pretty much didn't see the others during the next few years. So them meeting in high school is more like "ah, it's good to see so-and-so still doing well" rather than a more enthusiastic reunion for him? And then as they all start to hang out together again the memories come up, he recalls how fun it used to be, and he gradually gets closer to everyone again. 

...er, sorry for the wall of text lol.