Aozaru

青猿

The blue monkey residing within Kei's sword. In truth, he is a fragment of Youko's conscience that constantly teases her.
Aozaru (Azure Monkey) is the physical form of the sentient spirit inhabiting the scabbard of the Suigūtō (the Water Monkey Sword). This sword is a royal treasure of the kingdom of Kei and can only be wielded by the kingdom's legitimate ruler; consequently, Youko is the sole person able to draw it from its sheath. The blade was forged from a water demon defeated by a former great ruler of Kei, while the scabbard was crafted from a long-tailed monkey demon. A large jewel hangs from the scabbard, capable of healing whoever holds it.
The sword is not only able to kill demons and sages and break substances such as metal chains, but it can also present visions of the past, the future, and distant events. The sword and scabbard must remain together; otherwise, the sword's phantasms appear unchecked, while the scabbard's spirit is free to act as it pleases. When Youko loses the scabbard, it manifests as Aozaru, the blue monkey, and taunts her with her doubts, while the sword shows her visions of things she wishes to avoid. Eventually, Youko overcomes her doubts, kills the monkey, and retrieves the dead scabbard. Because she has killed Aozaru, anyone can now draw the sword from its sheath, although only Youko can wield it effectively.