Yuki & Zero

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

History
Yuki and Zero met at age 12 and 13 respectively, when Zero was taken in by Kaien Cross. Yuki immediately took to the young hunter and looked after him since the first night that she cleaned the emotionally fragile Zero's wounds. She promised to stay by his side forever after she discovered him distraught one night, tearing at his neck. Yuki would often stay up at night with Zero, patting his head after he had nightmares, helping him to fall back asleep. Yuki's nature overcame his distance and her presence also helped his nightmares to cease. Zero acted aggressively toward Yuki after learning of her love for the pureblood vampire, Kaname Kuran, possibly also because of jealousy. When Zero freaked out fearing his turning into a vampire, he accidentally lashed out and hurt Yuki. When Zero was attracted to the smell of her blood to his shock, he swore to the Chairman, that he needed to become stronger. Yuki accidentally admitted she was scared by Zero. When Zero tried to tell her his secret, she revealed her fear about being attacked by the vampire in order to empathize with Zero. However it caused Zero's anger to flare up about himself and caused him to try and push her away instead of telling her the secret. This was the beginning of Zero deliberately trying to put distance between himself and Yuki, yet Zero grew attached to Yuki and his affection for her grew, to the point where he refused to be adopted by Kaien because he did not wish to become Yuki's family. When Yuki went to see Kaname at the school and was shaken by the encounter, Zero observantly told her if she was not scared, she should not shake. He is very protective of her, which Kaien used to his advantage after Zero stayed back a year of high school and continued to refuse to enter until Kaien told him that Yuki would then be a school guardian all by herself.

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. In the course of the story, he repeatedly demonstrates that his remaining life is Yuki's. When Shizuka/Maria offered Yuuki a chance to save Zero from falling to a level E, she readily decides to give her own blood and humanity to Shizuka in exchange. Zero shows up in time to prevent the exchange from happening.

While dealing with his fall to level E, he has a nightmare about killing Yuki 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. Realizing soon after that her question has hurt Zero, she finds it inexcuseable and decides that she must do away with the uncertainty that led to it. 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'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.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 had killed him instead. Following a confrontation with Kaname, Yuki is able to talk alone with Zero. Yuki confirms that they have and Zero tells Yuki to go be with Kaname. At their parting, he states that the next time they meet they will be enemies and will kill her.