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. 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 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, during which he drinks her blood one more time and kisses her. At their parting, he states that the next time they meet he will kill her.

Post Time Skip
While confined to the Kuran mansion, Yuki comes to terms with her vampire nature. She eventually realizes that her thirst can't be fully quenched because a vampire's thirst can only be satisfied with the blood of someone they love and a part of her heart is attached to Zero. Their first meeting after a year occurs during the ball held by both vampires and hunters. They come face to face during the event, but do not act familiarly.

Their next meeting occurs not long after, during a visit to a graveyard. Yuki was attacked by Touma, a pureblood, ending up injured. Zero shows up and threatens the attacking pureblood to leave. Zero then takes the injured Yuki to a place where she can rest. Yuki is only half-conscious at the time and revels in his 'nostalgic scent,' nearly biting him. She wakes up fully before she does so, and hurriedly leaves with an apology. After the death of Hanabusa Aido's father, they meet again as Yuki chases after a rogue vampire who was kidnapping human children. They meet while capturing the suspect and exchange words, where Yuki internally asks whether Zero is alright. Zero, upon learning from Kaien Cross that Yuki intends to do something on her own, seeks out Aido. He informs the latter that she is planning something and that if he intends to help, he ought to speak either to her or Kaien Cross.