Kimi ga Iru Sekai

There's little difference between being alive and being dead. Either way is fine…anything is fine…

Before meeting Fuuka, Kuroo lived in a world without color. Through their relationship, he felt that he'd discovered the warmth in his own heart. However, an incident takes Fuuka away from him. Consumed by despair in a world without her, Kuroo follows a mysterious cat and ends up in another world—one where Fuuka is still alive…?

If I had never met you, if I had never lost you, if I could meet you again—what if life truly allowed such "ifs"…? A fantastical parallel-world love story.

(Source: Shogakukan, translated)