Kuzu no Honkai

Synopsis

From Crunchyroll:

We are the scum of humanity. Hanabi and Mugi are bathed in envious gazes from all around as a beautiful couple with excellent moral conduct. But these two apparently perfect people in an apparently perfect relationship share a secret they can't tell anyone.

Included One-shots:
Volume 1: *Nyan Nyan Prelude*
Volume 2: *Nyan Nyan Serenade*
Volume 3: *Nyan Nyan Oratorio*
The lonesome struggles of the only living being on Earth.
Volume 4: *Nyan Nyan Cantata*
A pet cat looks back on her life with her kind master when she feels that her days are coming to an end.
Volume 5: *Nyan Nyan Andante*
An abandoned cat grows fond of her new owner but she starts to feel insecure when the kind-hearted owner adopts another stray creature.

Synopsis & details from MangaUpdates