Yuki & Zero

History
Yuki and Zero met at age 11 and 12 respectively, when Zero was also 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 promises to stay by his side forever after she discovers 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 overcomes his distance and his presence also helps her 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 freaks out fearing his turning into a vampire, he accidentally lashed out and hurts Yuki. Zero is attracted to the smell of her blood to his shock, he swears to the Chairman, that he needs to become stronger. Yuki accidentally admits she was scared by Zero, when Zero tries to tell her his secret, she reveals her fear about being attacked by the vampire in order to empathise with Zero, however it causes Zero's anger to flare up about himself and causes him to try and push her away instead of telling her the secret. This is the beginning of Zero deliberately trying to put distance between himself and Yuki, yet Zero grows attached to Yuki and his affection for her grows, to the point where he refuses to be adopted by Kaien because he doesn't wish to become Yuki's family. When Yuki goes to see Kaname at the school and is shaken by the encounter, Zero observantly tells her if she is not scared, she should not shake. He is very protective of her, which Kaien uses to his advantage after Zero stays back a year of highschool and continues to refuse to enter until Kaien tells him that Yuki will 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 sacrificing her blood so that Zero can survive. Zero vows he will never let Yuki turn into a vampire.

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. Yuki then said 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. Yuki promises then to always run from him and the two part company. After she leaves, Yuki cries over being his enemy and apologises for being a vampire and declares that she will lock up her feelings inside. When she meets with Kaname, he tells her he knows that her heart is divided between the two boys.

After one year
Following the 1 year timeskip, Yuki remembers her vow to Zero to stay alive forever. Yuki finally admits to Kaname that part of her heart belongs to Zero and that she cannot be satisfied without his blood also and she cannot sever the link even after a year.

Yuki meets Zero at the peace party. When she touches him, however, Zero asks her to remove her hand away in disgust, however, both later gaze at their arm/hands where they touched and Zero feels a surge of blood lust. Aido invites Zero to meet with Yuki and Yori, but Zero declines. When they cross paths again at the party, Zero tells her off for trying to interfere in the vampire hunter investigation which spurs Yuki to declare she will find out the truth to protect her loved one.

Yuki's mission
A little time later, Yuki arrives at the cemetery to visit her former caretaker, but doesn't go in as Zero is already there. After Yuki is badly wounded by another pureblood, Zero comes across her. Following her collapse, he picks her up and takes her to a Vampire Hunter safehouse. Yuki awakens while still being carried by Zero, in her semi conscious state, she recognises the smell as nostalgic causing her to almost bite his neck. Zero does not fight it and continues to hold her. Flustered, she later lets him go and apologizes then jumps out of the room's window.

In the Anime
During the last episode of Vampire Knight Guilty, Zero's hug, bite and kiss does not occur, instead Zero asks her to drink his blood to prove that she really has become her vampire self. Zero is not surprised, but instead vows that one day, he would kill all purebloods, including Yuki.

Their Relationship
Zero's tsundere and agressive nature keeps him from befriending people, ensuring Yuki is the only one close to him. Yuki's sunny, stubborn, and silly nature allows her to ignore Zero's aggression and rudeness overriding his desires with her own, inadvertently bullying him. Zero's affection for Yuki causes him to fiercely protect her and be easily directed by her. His nature is primarily shy and he does not communicate his feelings towards her, though he expresses frustration in the form of bullying her at times, causing Yuki to think he didn't like her earlier in their history. He refuses to allow Kaien to adopt him, because he doesn't want to be related to Yuki, though when she expresses curiosity, he ends up being very rude to her.

Not until 20th Night, when Zero asks Yuki why she does so much for him, does Yuki begin to actually think about their relationship and how she views Zero, though no conclusion is shown from this. Yuki was almost completely blind to Zero's affection for her until 25th Night when he almost kisses her, it is speculated that from this point that she begins to see more in Zero. She takes his friendship for granted and becomes very reliant on him, she remains somewhat ignorant of his feelings, causing her to confess her love for Kaname in front of him on more than one occasion.

Following Zero's kiss in 46th Night, Yuki is finally made aware of Zero's feelings for her. Yuki confirms in 47th Night that she finally understands his earlier overtures, but her status as a vampire means they cannot be together even as friends as it hurts Zero too much, she reveals that she will lock up her feelings for Zero in her heart.

Yuki finally admits she feels something for Zero in 51st Night, but it is left ambiguous as to how it fits with her feelings for Kaname, only that she feels it is unfair on Kaname and should not be allowed to still stay at Kaname's side. Yuki continues to be protective of Zero's feelings and notes in the party how Zero will perceive others like Sara as his enemy and she later notes his pain on seeing the vampire hunter girl having been turned into a vampire.

Following Yuki's revelation of being a pureblood vampire, Zero's feelings of anger have been more obvious than his love for her. After the year timeskip, his love and bloodlust for her is left ambiguous. Both characters avoid calling the other by the names they once so comfortably used. Despite Yuki's ambiguous feelings, their relationship has been reset somewhat from the comfortable friendship because of their status as enemies, Yuki's awkwardness is on display when during a coincidental meeting in 60th Night, Yuki greets Zero with a "Good evening" and mentally berates herself for being unable to respond better.