Marshmellow is the head of a gang of thieves known as Mitsukubi Condor, and he is obsessed with beauty. One day, a girl named Cocoa comes to him, trying to find the harp that was precious to her village. He decides to help her recover the valuable harp, which plays beautiful music that can draw out rainbows!
(Source: MangaHelpers)

Included one-shots:
Volume 1: Ymir no Majin
Volume 2: Parasite B
Volume 3: Mitsukubi Condor (pilot)