Maybe Roku ended up with à Guy because he feels so guilty about Kei that he's started to behave like him at à certain extent, he bottoms like him and he triés to fil the hole he feels by being f by a Guy like Kei was doing before him. He goes to the rooftop and he prays for what? Maybe he prays to not being completly drowned by the souvenir and the flashback and this réflexion he saw everytime he looks in the mirror or even at normal Times. The Guy he is with srems to know à lot about Roku and even srems to care about him we can also see him be angry and jealous in a way of the photograph because with his pictures and his behaviour toward Roku, Roku starts to feel something, and he even talked about the Guy when he was with the "mean" one and it didn't sit we'll with him.
What happened to the photograh I don't know but we can imagin him and Rijo becoming Closer because he's able to anderstant what suicide of à close relative means and what it entails he can understand and then maybe he can save Riko from the hole he started to dig for himself.
But again it's just my Guess haha I hope one day we will know the last Word of it.
When I saw that Psychological tag I should have just moved on. Not to mention I felt that there was too many questions unanswered. How did Roku end up in a relationship with a guy. what happened to the photographer student?
I'm going to have to make up a happy ending for my self to get over this.