Rascal Does Not Dream of a Dreaming Girl (light novel) — Hajime Kamoshida

SUPERSTRING (OF FATE) THEORY! Sakuta is doomed. His girlfriend, Mai, has just discovered that while she was away, he's been living under one roof with his first love, the now-college-aged Shouko. While the three of them try to navigate this complicated love triangle, the mystery surrounding Shouko only deepens when the younger version of herself checks into the hospital. What will become of this middle schooler whose only dream is to grow up? Why did Sakuta's old flame come barging into his life again after all this time? And exactly how does Sakuta fit in?