Maou Shoujo wa Pancake no Yume wo Miru

Otani Yorika (14) was a supporting character in his own life. After living such miserable days, Yorika committed suicide, only to find that an unexpected fate awaited him: becoming the demon king's pet in another world!?

"I'll defeat the demon king, gain my freedom, and finally become the protagonist of my own life!"

Yorika made this decision, and then a tiny demon girl appeared before him, with sturdy sheep's horns, moonlight-colored hair, and sparkling crimson eyes.

(Source: Kadokawa, translated)