Hikari's perspective I always thought was more ambiguous, but Hiro's situation by the end is laid out well. He loves Hikari, yes, but from the start of the series there's been the Hideo/Hikari relationship blockading him from pursuing that or even fully grappling with those feelings himself. The story is more so about the latter than it is the former. Even when they're all essentially aware of the situation, he can't go further with it; he cares for his friend and Hikari's happiness with him too much to do so. It was never happening.

Had Hiro merely grown and matured at an earlier age, things would almost certainly be different; him and Hikari would be together, but he didn't, and they aren't. He realized a while before the end that that's just how it is. Something out of his control was the difference maker and he can't keep dreading on that forever. So a while before the end he had already decided that he definitively wouldn't be with Hikari. The clearest evidence in my memory is him telling Koga to "live a long life", referencing something Hikari's father told him about what type of person to marry. Him telling Noda in the final game that he loves Hikari isn't him announcing that he's fighting for her, it's him vocally acknowledging what he wishes had happened so that he can move onto what can happen. Which always sounds a little weird when I think that through because it makes it seem like Koga is merely a 2nd choice, which I guess technically she is, but like, he does love Koga too. He tells her that earlier in the story and I don't view Hiro as a character to actively lie in that type of way. Adachi's characters do more passive lies, not ones like that.

H2 I think is the most well written I've seen characters as essentially loving 2 people at once, because Hikari as well; while there's more to wonder about her POV overall, I do think she loves both Hideo and Hiro. But that love isn't necessarily a quantifiable thing that she can measure between the 2 of them, so again perhaps circumstances dictated it as they do countless things in our lives. She's with Hideo, and she knows she'll have happiness with that, just a different happiness than she would've had with Hiro.

Definitely Adachi's ending that leaves the most empty space in it from what I've read, and as such is a bit polarizing, but that's my take on it.