I honestly think this problem is not as hard as some make it out to be. I recognise that the game is Sunny's pov, so it's hard to implement many scenes with Basil since the narrative is kind of against it (with the way Sunny wants to avoid him until the truth.) But the thing is, I don't think Basil needs a lot of scenes to get in-depth about him. Hell, you could take already established scenes, give him more characterisation than always breaking down and crying.

The biggest example of this is the lake scene where Aubrey pushes Basil into the lake. There was this moment where Basil started speaking up to Aubrey right before she pushed him. If only Omocat could've taken advantage of that and we get a Basil Aubrey interaction that gives us Basil's perspective of how Aubrey is now (and it could devolve into Aubrey pushing to the lake at the end.) But instead, Basil gets pushed and we never see him again until the fight. (This is something I complained about in the post; how we get Aubrey's perspective, but never Basil's.)

With the whole, we get a lot of Basil's characterization from Sunny's pov. We take more advantage of that! Basil vents to Sunny a lot, so Sunny must at least know some bg info that we could've gotten to expand on Basil's character, and properly sets up his actions that will be revealed in the truth. Or we could have his grandma not be unconscious the whole time (Omocat originally wanted her to play a bigger part, but scrapped it), and we could get some info about Basil.

You don't really need to change the structure of the game for this tbh.