Yuki & Zero

History
Yuki and Zero met at age 11 and 12 respectively, following the attack on Zero's parents. 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. Zero acted aggressively toward Yuki after learning of her love for the pureblood vampire, Kaname Kuran. Yuki's nature overcomes his distance and his presence helps her nightmares to cease. Zero stays back a year of highschool, refusing to enter until he is convinced by Kaien to attend in order to protect Yuki from the night class.

Manga Summary
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.

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 and apologises for being a vampire. When she meets with Kaname, he tells her knows that her heart is divided between the two boys.

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. She however does decide to stay by Kaname's side forever.

Yuki meets Zero at the peace party. When she touches him, however, Zero asks her to remove her hand away in disgust.

In the Anime :]
During the last episode of Vampire Knight Guilty, the reconciliation does not occur, instead Yuki drinks the blood of Zero to prove to him that she really has turned back into a vampire. Zero is not astonished, but instead vows that a day would come when 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.

Yuki was blind to Zero's affection for her until chapter 25 when he almost kisses her, it is probable 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 completely ignorant of his feelings, causing her to confess her love for Kaname in front of him on more than one occasion. Yuki finally admits she feels something for Zero in chapter 52, but it is left ambigous as to how it fits with her feelings for Kaname, only that she feels it is unfair on Kaname.

Following Yuki's revelation of being a pureblood vampire, Zero's feelings of anger have been more obvious than his love for her. Following the year timeskip, his love and bloodlust for her is left ambiguous.