Oh boy. I think Rui would definitely start avoiding Tsukasa. These two don't talk about their feelings as much as they should in the first place, and Rui in particular doesn't even realize he's feeling a certain way half the time without someone straight up telling him. He'd feel so incredibly guilty and start spiraling out of control, thinking if he didn't come up with this idea, if he didn't suggest this play, if he didn't become Tsukasa's director, then none of this would have happened. The worst case could have been than Tsukasa got hurt in some sort of life changing manner, and the thought that he could have been the one to snuff out his star's future would be almost too much to bear. He'd probably start saying something along the lines of "I'm unfit to pave your way to stardom" or "I'm a failure of a director for letting an actor under my charge get hurt." He wouldn't be able to face Tsukasa or himself, the guilt would be overwhelming. It's basically his worse nightmare, because all of his insecurities from childhood would just come back full force. Those kids would have been right to shun him, since what happened to Tsukasa would prove that not trusting him was the correct choice.

I do like the idea of Rui becoming obsessed afterwards though, almost like having Tsukasa withdrawal symptoms. He certainly wouldn't have the guts to watch over Tsukasa by his side, and he'd probably think that Tsukasa is the safest away from him, but watching him with cameras is a whole other story. In this case Rui ends up moving transferring away from Kamikou because he doesn't want to face Tsukasa. (He has a history of doing this and I can't see him avoiding Tsukasa forever if they did still attend the same high school)

It starts out as just wanting to make sure he's adjusting back to his daily routine, that he's truly okay and that there's no complications. Soon it turns into watching him in every free moment he can, wanting to know what he's doing at any given moment. He even ends up watching Tsukasa sleep instead of sleeping himself. And of course, the jealousy that builds within him every time he catches Tsukasa talking to Toya or Akito. Rationally speaking Toya is probably just asking after Tsukasa's health, but Rui wishes it could be him that helps Tsukasa up the stairs or carries his bag for him.

Rui grows steadily more obsessed with watching over Tsukasa, to the point where he programs his drones to take pictures and record audio. If he can't be there to make memories with Tsukasa then he can capture the memory himself and fantasize about it. Whether Tsukasa notices or not is up in the air, but I imagine he would be worried for Rui as well. They've already gone through this once before back during their Halloween event, so he knows Rui must be taking this hard. He probably also misses his presence as well, having spent so much time hanging out together. Sometimes he'll catch himself calling Rui's name by their shoe lockers or walking up the stairs to an empty rooftop. It's noticeable to the people around him as well, how his enthusiasm has dimmed and he isn't quite as boisterous as he used to be.

Fun thought though, Tsukasa misses Rui so dearly that one night he pulls out an old sweater Rui left one time when he stayed over and hugs it to sleep. He never got the chance to confess his feelings before all of this happened, and while he's been putting on a strong face so his friends and family don't worry, he misses Rui so much he can't bear it. Cue Rui tuning into the camera stream he has set up in Tsukasa's room and seeing him rubbing against his old sweater while mumbling Rui's name with watery eyes. I'm sure that sight would be a wake up call for him (and his dick too LOL).