Jungle wa Itsumo Hale Nochi Guu

Synopsis

The story is about Hare, a 10 year old boy who lives with his mother Weda in a small village in a very strange jungle. Hare's mom is a bit of a "party girl" and likes to sleep in all day and party with her girl friends all night. Things change one evening when Weda comes home with a little "playmate" for Hare, a pretty little girl named Guu. But when Hare wakes up the next morning, Guu has "transformed" into a sullen, nasty, and sarcastic joker who takes great pleasure in making Hare's life utterly miserable. Guu can also swallow anything and regurgitate it whole. Guu also seems to have various magic powers, which lead Hare to consider her a demon of some sort, but he can't convince most other people of that.

Synopsis & details from MangaUpdates