From MangaHelpers:

8 years ago, Hirasaka Itsuki often went to the mansion of a certain girl to play with her. Her name was “Riri.” But one day, there was an explosion within the mansion where Itsuki and the girl were. Although Itsuki was saved, with many burns and injuries, the girl was reported as dead.

Now in the present, Hirasaka Itsuki has been absent from school for three days, for reasons that would not make any sense to any regular person. Ever since 3 days ago, an apparition of Itsuki’s past has appeared before him in the form of the girl he once knew, but no one can see this apparition. This apparition now attempts to attack Itsuki, but who comes to his rescue? It's the girl who was thought of as dead since 8 years ago: Riri. What relation does this apparition have with Riri? [Midori-Mori]