Kagerou Project is a collection of Vocaloid music works produced by Jin (Shizen no Teki-P) — which first began with the release of Artifical Enemy on February 17th, 2011. Since then both a novel (Kagerou Days -In a Daze- — illustrated by Shidu), and manga series (Kagerou Days — illustrated by Satou Mahiro), was started to connect all of his songs together.
For those wanting a translation of the manga, you can find something here.
*Children Record PV:
*Song list (Kagerou Project):
(From left to right) Momo, Kido, Shintarou, Ene, and Hibiya
(From left to right) Hiyori, Kano, Konoha, Seto, and Mary