Title: Futtemo Haretemo (Come Rain or Come Shine)
Mangaka: Fujimura Mari
ISBNs: 4086180251, 408618026X, 4086180278 (this is the reprint; the original printing was 5 volumes)

A shoujo yuri manga from the 90s. When Hiro transfers to Nagi's school, the two of them become fast friends, though Nagi seems to be the only girl in her class who likes Hiro. That is, until Hiro sets Nagi up and turns the rest of the class against her. They reconcile, and continue to be friends... but when Nagi finds Hiro's room plastered with pictures of herself, she can't help but wonder if there's something more going on. Unfortunately for Hiro, Nagi has a boyfriend, and Hiro has some problems of her own in that department...
See also Okazu's review.
Many thanks to Bee Q for cleaning these scans for me!

Volume 1

Volume 2

Volume 3 (first half only, so far)