I think tsubaki is feeling very guilty for what he did to yuu and he feels like he have to take responsibility for that and he doesn’t not how to do it. As a child tsubaki way of thinking is very simple, he thinks if he can be the most important person to yuu, the one to be there for yuu and for yuu to replay on, it will make up for what he did.
Yuu,however, doesn’t see tsubaki other than a little brother and he still sees tsubaki as a child that he used to take care off. He doesn’t take the things tsubaki say seriously. He see tsubaki as a child, even though tsubaki is in fact a teenager,and just go along the flow to make bc he thinks tsubaki doesn’t know better and it also makes tsubaki happy. What how I see. I don’t see romance here, I see two people that deeply care for each other, that don’t include love in the romance way.
I never liked the seme from the beginning. Hope he never comes back. I also hope that the uke doesn’t give in so easy to the seme.