Aozaru

The blue monkey that dwells inside Kei's sword. Actually he is a part of Youko's conscience who is constantly teasing her.

Aozaru (青猿, Azure Monkey) is the physical manifestation of the sentient spirit of the scabbard of the Suigūtō (the Water Monkey Sword). The sword is a royal treasure to the kingdom of Kei and can only be wielded by the kingdom's rightful ruler - as such, Youko is the only one who can draw the sword from its sheath. The sword was forged from a water demon that a previous great ruler of Kei had defeated while the scabbard was created from a long-tailed monkey demon. From the scabbard hangs a large jewel that can heal the one who holds it.