Beheneko: The Elf-Girl's Cat Is Secretly an S-Ranked Monster! - S01

While carrying out his sworn duty to protect humanity, a young knight dies in battle against demons. Fate intervenes, and he wakes in the deepest level of a dungeon, reborn as a Behemoth, an S-class monster of unmatched strength and one of the very creatures he once vowed to shield people from. As a juvenile, he looks just like an ordinary orange cat. Though he has the seemingly overpowered ability to absorb the skills of any creature he devours, his escape from the dungeon leaves him badly wounded. Just in time, Aria, a D-rank adventurer, finds him, decides to nurse him back to health, and ends up taking him in as her pet, naming him Tama. Recognizing Aria as his master and with his knight's honor from his past life on the line, Tama swears to protect her from danger no matter the cost, even if it means throwing away this second chance at life.