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!

Aimu Yuu (remark: his name sounds like "I'm you") is a high school student who just moved into Yaninari Sou, a cheap boarding house in Tokyo despite the rumor of this place being haunted by ghosts. Upon his arrival, he finds out that he's sharing room with a female ghost named Mii, who immediately takes a liking to him and insists on clinging to him till he dies. Not only that, but Yaninari Sou is filled with weirdos like Hinata Teruaki, an antisocial guy, Gouda Karuma, a 8-years ronin with luxuriant hair, Ogura An, a weird Shoujo Mangaka, Buraide Mari, a former pro wrestler, and last but not least, the mysterious landlord Ooya Oya who's always wearing an armor suit. The only normal person Yuu can find here is his classmate Ooya Ai, the landlord's daughter, and she also seems to take a liking to him, but will this be enough to help Yuu get through his high school life in peace? The title "Yuu & Mii" is a pun on the English phrase "You and Me". A parody manga with a lot of weird but enjoyable characters.

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!