Yukina was sent to observe the fourth primogenitor (aka Kojou) and was told to kill him if he becomes a threat. She is constantly following him everywhere he goes and as a result has become very close to him, even allowing him to suck her blood in order for him to gain control over his familiars. Unbeknownst to her, she was actually sent for this very purpose; with the organization intending for her to be his first blood partner, and even his mate.. By the end of the first arc, she seems to have developed feelings for Koujo. In Strike the Blood EX, she is married to Kojou and they have a daughter named Reina.
She's Yukina's childhood friend, ex-roommate and a War Dancer from the Lion-King. She is over-protective towards Yukina.
Nagisa is Yukina's classmate and close friend. She is usually chatty with people she likes, including Yukina. When Yukina learn the truth past from Kojou about Nagisa false past, they decide not to telling her about this. And decide telling the truth to her when the time has come.
Is Yukina and Kojou's daughter. She has given Yukina advice for the future which Yukina takes on fully.