Yuki & Zero

Yuki and Zero are childhood friends who made excellent working partners balancing each other's nature out. Supportive of the other until the moment Yuki became a pureblood vampire and Zero declared her his enemy.

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. 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. Zero bites, kisses and finally hugs Yuki, after Zero confirms that Yuki is ok, he confirms that the next time they meet he will kill her.

1 year later
Both Yuki and Zero have carried on with their separate lives and not seen each other. Yuki is struggling with her hunger and finally after much self torture, she admits to Kaname that she has a bond in her heart to Zero that she has not been able to break and that Kaname alone cannot satisfy her hunger.

Yuki and Zero have a clash at the vampire party, when Yori searching for Yuki is found by Sara. Yuki is slow to respond and find Zero manhandling Sara and she tries to intervene while laying her hand on Zero. Zero refuses to let go of Sara until Yuki stop touching him, to which she complies. When Aido invites Zero to join Yori in a private visit with Zero, he declines.

When Yuki visits the graveyard and is attacked by Toma, Zero senses her presence and investigates only to find her being attacked, he threatens Toma and makes him leave. An injured Yuki feigns wellness, but collapses, thus Zero scoops Yuki up and takes her to a nearby hunter safe house to recover. Before he can set her down, Yuki awakens and smelling his nostalgic scent she almost bites him to her distress. She frantically departs and tells Kaname he can punish her.

Zero's prickly attitude toward Yuki is noted by Aido who taunts Zero, telling Zero he should go after what he wants. Aido later retracts the statement, but advises Zero that Yuki is doing her best. The sentiment is later echoed by Yori who also notes that Yuki hasn't changed.