The Three-Body Problem

Lilian Eddington finds a mysterious but cute black beast in the woods, which she refuses to abandon when her grandfather says she can't keep it as a pet.  After he finally consents, she kisses the newly named Woolbo on the nose only for it to release the seal on it and it turns into the Maou. While condescending at first, the Maou decides he can't abandon Lilia either.  How will their days together play out in the future?