Mugenkūchū is one of the most unassuming, yet deadly melee type zanpakutō known. With the name meaning "Infinite Sky", Mugenkūchū is owned by Head Captain Saito. Mugenkūchū takes the form of a young child with blonde hair and headphones, but still boasts immense power on its own. Upon activation of Shikai, Mugenkūchū takes the form of a wooden training bokken, with the phrase "Yūjin to teki" written on the hilt. Mugenkūchū here has the power to cut through any physical matter, as well as blunt itself to apply deadly bludgeoning force.

In Bankai form, this spirit becomes Mugenkūchū Danzetsu, Infinite Sky: Extinction. In this form, Mugenkūchū looks much older as a spirit, still looking human but more athletic. The appearance of Mugenkūchū changes to a long nodachi with a golden dragon as the tsuba. At this point, Mugenkūchū has the ability to cut through the non-physical.

Mugenkūchū's final form is Bankai Fusion, the merging of Arata's soul with that of his spirit. This form is called Mugenkūchū Shiroakuma. As Mugenkūchū increases in power level, it channels power in between itself and Saito. However, this transference slowly seeps away Saito's humanity in the procees. In its final form, Mugenkūchū grants Saito the ability to become so numb that he doesn't even register pain. This makes him unstoppable, but at the same time, takes away his humanity completely, making him unable to distinguish friend from foe.
--coded by Nicole--