Yuki & Zero

Yuki and Zero are childhood friends and met at age 12 and 13 respectively, when Zero was taken in by Kaien Cross.

Zero's awakening
When Yuki discovers that Zero was bitten by a pureblood vampire turning him into a vampire too, she began encouraging Zero to drink her blood so that Zero can survive knowing its against the rules. While dealing with his fall to level E, he has dreams about killing Yuuki and almost kisses her after awakening and promptly denies his intention citing confusion. During a moment of weakness, she wonders if becoming a vampire will make her stronger and Zero vows he will never let Yuki turn into a vampire. She desperately searches for the answers to her past and Zero tries to help her in her search, going as far as confronting Kaname on her behalf. He eventually ends up drinking Kaname's blood to his immense distaste in order to cease hurting Yuki with his hunger.

Yuki starts falling into her own madness and almost strangles Zero, when she asks him why he didn't try to stop her, he states that his life belongs to her. She berates him and states that she has been using him as a distraction from her own problems.

Yuki's re-awakening
However, later when Zero discovers that Yuuki is actually a pureblood and Kaname's little sister, Zero is angry. Following the exchange, Yuki desperately wanted to see Zero and thus ran away (Hanabusa Aido and Akatsuki Kain had to follow her for protection). When Yuki was about to knock on Zero's door, a gun was pointed at her by Zero, declaring that all he senses behind the door is a controlling pureblood who tampers with peoples lives. Yuki then responds saying that the old Human Yuuki that Zero knew is now gone, and that she has been eaten up by the vampire Yuki. Zero and Yuki meet each other again confronting Rido, Yuki desires to ally with Zero, but he declares her his enemy. The two however end up fighting together to defeat Rido, after which Zero points his gun at Yuki and declares his intention to end it all after killing all pureblood vampires including Yuki. Zero is unable to pull the trigger as he remembers the Yuki of his past and sees Yuki with the same sincere and unwavering gaze that always gave him strength. He then relents that he wouldn't have minded if Yuki has killed him instead.

Following a confrontation with Kaname, Yuki is able to talk alone with Zero, he confirms that the next time they meet he will kill her.