Tennōseiga (被験者の皇帝牙) is the name of the synthetic organism which has bonded with Naoto and serves as his replacement arm. This was gained through unknown means by Harou medical ninja and seems to have been grown from the flesh of a tailed beast. Due to the organism, Naoto has gained access to the Water Release, however it is not originally his power so it causes him great strain to manipulate the nature properly, requiring more focus and skill than his other natures.