KazuKouHiro but it's the Love Triangle to Polycule trope π³
Like Hiro loves Kouji (this one is obvious I don't have to explain it) Kazuki loves Hiro (Is the one to save Hiro in PRRL + the Road to SSS arc he really wants to be his partner etc.) and Kouji loves Kazuki (Harder to justify but childhood friends, they're each others confidant, they were together when Hiro took the stage during Pride the Hero they definitely were talking the whole time.)
I have a sillyβ’οΈ headcanon that when Hiro and Kazuki first talk in Hiro's room in PRRL Hiro tells him about his crush on Kouji, this works here because it's the catalyst for the triangle. Kazuki now knows about this, and vents his own crush to Kouji. Kouji probably doesn't tell Hiro about his crush through because like he's not stupid he definitely can tell Hiro likes him, he just doesn't know the extent or if Hiro is homophobic or whatever.
Anyway they have all this love triangle tension, and one day one of them, probably Kouji he's the smart one, gets hit with the realization of like hey, we should all just date each other. Boom.
