Shibito no Koe wo Kiku ga Yoi

Jun Kishida is a sickly high school student who can see dead people. One person in particular is following him around: his childhood friend Ryoko Hayakawa. Even after he uncovered the truth behind Hayakawa's death, she stayed by his side. As he and his classmates get into even more strange and mortally dangerous situations, Hayakawa's spirit quietly aids him.
(Source: MangaHelpers)
Included one-shot:
Volume 4: Yumi-chan Kitan