Same Cell Organism (2001)

Blurb:
Excited, angry, calm, stressed, happy, sad, warm, exasperated... Feelings that come one after the other, or even perhaps all at the same time, when one is involved in a relationship. These are also the same feelings one will encounter one after the other, or perhaps, even all at the same time on the pages of Same Cell Organism. Stories included in this collection of carefree vignettes involve an angel who becomes mortal to be with the one he loves, two loners who share a fondness for hiding out in the school attic, and a story about a high school boy in love with a college man who seemingly sees him as nothing more than the kid next door.

Source: www.anisearch.com/manga/1413