Your & My Secret(2001)

Boku to Kanojo no XXX

Two teens see how the other half lives!

The girl: cute but obnoxious, pretty but violent, petite but rude.
The guy: shy and slender and secretly in love with the girl.

When her mad scientist grandfather accidentally switches their bodies--it's freaky Friday, Saturday, Sunday, and every day! Akira Uehara is mortified when his best buddy starts hitting on "him," and is embarrassed to look at "himself" in the mirror in her underwear! He can't wait to get back to being a boy. But Nanako Momoi has other ideas--she is starting to enjoy life in Akira's body!

(Source: ADV)