At Yumenosaki he kept his past a secret for a long time and stood out as a rebellious outsider and a loner. Doing what he wants instead of what he should tanked his grades, and he was held back, constantly finding himself the oldest student in any group and always struggling to fit in.
He jumped between units, not settling anywhere, but becoming aquainted with many different people. Him and Keito had a sort of student council/delinquent personal rivalry.

I think An would be only a background character at Yumenosaki, but he'd always be hanging around seemingly everywhere except class, or teasing Keito and cheering the cast on from the sidelines.

The first time An really noticed Keito is when he ended up at the Deadmanz performace of ROCK ROAR in that one underground club. He became instantly intrigued by this guy who's obvoiusly the most uptight in this improvised rock group and is still putting in his all, with the effort showing on his face.

After this An started occasionally visiting Keito, tagging along for student council duties (Keito did not ask, this guy doesn't even help most of the time, what the hell? Rude and annoying. In Keito's opinion.), and taking naps in close proximity.
From Keito's perspective he's just gained an annoying stray cat (who he's never even exchanged words with before), who loiters around and teases him at inconvinient moments, and he can't get rid of him. And if all this wasn't offensive enough he keeps finding An in places he isn't supposed to be and has to chase him back to class with a broom.

Eventually Keito begrudgingly accepts An's presence, often taking time to lecture him on literally everything. (It's not like he kind of enjoys An's wierd one-sided friendship nono he would never)

He continued with his relaxed attitude even after graduating and not finding a unit, he also was under threat of bieng fired and was grouped in with the future Alkaloid members as an underachiever.

At first he was very distant with his bandmates, but the threat of bieng fired and not having stood on a proper stage even once eventually drove him closer to them. He is one of the units older members, together with Mayoi and Tatsumi, and found himself feeling very protective of Aira and Hiiro, wich was a first for him.

His biggest challenge was learning how to not be afraid to give his all to something, be it performance or friendships.

An bieng held back meant Keito graduated a year before him, and they lost contact as Keito's career took off. He felt wierd about it, but justified it as them never really bieng friends in the first place.

A year later he accidentally saw An's name in an info post about the Ullambana Festival, and acting on a nostalgic whim he went to see the show. He had some feelings he'd rather not discuss.
A month or two later An (with his bandmates encouragement to socialize and open up) showed up in the manga club. Where Keito also goes to. Keito had some feelings about that.

AAAAHH i got so many words about them FUCK
But slowly Keito and An start hanging out in earnest like normal people, having grown up and gotten over some bullshit, and kind of get crushes on each other and start DATING and it's GREAT!!! YES!!!! YAOI WINS!!!!!!