I'm A Behemoth, An S-Ranked Monster, But Mistaken For A Cat, I Live As An Elf Girl's Pet (Manga) Vol 03 (Mature)

Aria and Tama are heading straight toward a battle against a Demon, and much between them will have changed by the time it is over. And why did Aria choose to leave her elven village to become an adventurer...? Yen Press
(W) Nozomi Ginyoku (A) Mitsuki Yano