read X by the same author.
its not about them, but they will conclude their story there.
Do you know which chapters?
Subaru is one of the main supporting cast, and seishiro is one of the main supporting villains, so stuff regarding their relationship are in multiple parts in X, but the big part those two have together isn't until close to the end.
X is good, so, if you start from the beginning, you might like it.
also, you might be confused about what's going on if you skip stuff.
Still, a It's really good manga!
Didn't everyone die in X????
*spoiler* in the manga Seishiro get killed by Subaru but Subaru is still alive. Unfortunately Clamp never completed the Manga series. (I‘m still waiting and hoping since 2003)
What. The. Hell.
THAT was the ending?!!?!?!??
How? What just happened? I need closure. They should be in love. IN LOVE. They were cute together and even though Seishirou is messed up, he could have been messed up and still in love with Subaru. This is not okay. I am not okay with this ending AT ALL. Sequel please. Please write a sequel where one of them dies or falls in love or both if you want to keep the bittersweet mood up. But PLEASE give me some actual closure. I'm dying here.