She is Minato's fourth Sekirei and her power is water (derived from her name "umi" which means the sea). She contacted him over a dream saying that she would kill him, but when they finally met she was persuaded into becoming his Sekirei. (chs. 22-23) She says that she is Minato's legal wife. At the beginning, she wanted to be the strongest to demonstrate that Ashikabi were not necessary to win but later she confides to Miya that her only goal is to get stronger to protect and live together with her Ashikabi forever.
Tsukiumi is a Sekirei with a tsundere personality and the most aggressive of Minato's party and has a very short temper; she rarely smiles. She was introduced as the first Sekirei that wanted to become the strongest without the help of an Ashikabi. She even stated that she was willing to kill her future Ashikabi if she would ever react to him/her, due to her hate towards Ashikabis. She changed a lot after getting winged by Minato and easily gets jealous when Minato comes in contact with other women, due to her feelings for her Ashikabi. She proclaimed herself to be Minato's legal wife which caused a rivalry between her and Minato's remaining Sekirei, in general Musubi with who she often competes to gain the right to be called 'Minato's legal wife'. Aside from this, she gets along well with Minato's other Sekirei.