Beheneko: The Elf-Girl's Cat is Secretly an S-Ranked Monster!

Synopsis

A knight dies in battle against the forces of evil and is reincarnated as a behemoth, an S-ranked monster of immense power, waking in the depths of a dungeon in the labyrinth city of Ryuin. The catch is that a juvenile behemoth looks just like an ordinary orange house cat. Badly wounded after escaping the dungeon, he is found by Aria, a beginner D-rank elf adventurer, who nurses him back to health, mistakes him for a kitten and adopts him as a pet named Tama. Moved by her kindness, Tama swears to protect her with the honor of his former life as a knight, using his formidable strength and his ability to absorb the skills of creatures he devours.