Abgrund Family[]
Ein Abgrund[]
Ein is Yui's older sister. They were both trained to become deadly assassins. Later, Ein killed the entire family, which caused Yui to chase after her. This was not because Yui wanted revenge for her family, but to redeem their reputation. Yui seems to despise Ein. And yet, during their battle, Yui reminisces of the time when she and Ein were on good terms with each other and she considers her older sister to be a model at the time and those memories gave her the strength to keep fighting and ultimately overpower and kill Ein, although before doing so she thanks her and cries a bit after dealing the fatal blow.
Hagun Academy[]
Stella Vermillion[]
Yui and Stella often tease each other, though they have developed a friendly relationship. Yui initially faced Stella in the Festival, feeling that she was too arrogant towards her. Later, Yui joined Team Vermillion, after saving Stella's parents. Yui taught Stella a better way to fight, helping her develop. Stella, though still sometimes unfriendly towards her, has given respect to Yui.
Akatsuki[]
Rinna Kazamatsuri[]
Yui was annoyed by Rinna's magnificent speeches and childish attitude. Rinna viewed Yui as inferior to herself.