The title "Yuu & Mi" is a pun on the English phrase "You and Me". Aimu Yuu (remark: his name sounds like "I'm you") is a boy who just moved into the equivalent of a boarding house.
Everything is not as it first seems, even the boy. Living in the same room is a ghost (female) named Mi. The series is a combination of love, humour, and a general parody of anime and manga. There are a lot of weird characters in this manga.

A young high school couple out on a date saw what looked like a falling meteorite, and inside, they discovered a little baby girl! The two of them married and adopted the girl, who they named Morumo, as their own. The only problem with this feel-good story is that now, as a high school freshman, Morumo is 16.4 meters tall!

Morumo still has a pretty happy life. She's popular with locals and tourists alike, and she goes to school everyday with her childhood friend Miharu. Then one day, a mad scientist, who specializes in experimenting on young ladies, injects Morumo with a serum that reduces her to 1/10 of her normal size. Her weight remains the same, so she is super-dense and tough like a superhero. Suddenly, it seems like all the joys of an ordinary life are within her grasp!! Sadly, the serum's effects are temporary and soon Morumo returns to her giant state.

But now that she knows a normal life is possible, Morumo is not going to give up! Even if it means she and Miharu have to allow a mad scientist to experiment on her, surely the ends will be worth the means!

If you come by this girl, you better be extremely careful - nobody knows what kind of prank she will play the next moment! She practically lives to cause mischief!

From Wikipedia:

Shimeru Futaba is a normal high schooler living a normal life, active in his school's wrestling club and slowly getting closer to his awkward love interest, Misaki. This fails to last, as he discovers his family's hereditary genetic defect that becomes active at adolescence. Although it will eventually become controllable, excitement and stress now make Futaba switch sex. Hilarity ensues.