Zero kept his vampire secret from Yuki until bloodlust overwhelmed him and he bit Yuki. Zero attempted to run away, stopped by Yuki, who pledged to kill Zero should he fall to level E. Following Zero's awakening as a vampire, his tattoo was used by Kaien Cross to allow Yuki to tame Zero. Zero's body rejected the blood tablets used to sate a vampire's thirst, thus Yuki convinced Zero to continue drinking her blood. Zero is still depressed and almost allows his former teacher, Toga Yagari to kill him until Yuki intervenes.
Zero reveals he can sense Yuki's feelings and accuses Yuki of preferring to give her blood to Kaname. Yuki chases Zero when she thinks he is running away, but finds Zero has been sent on his first vampire hunter assignment. Yuki is caught by the vampire, but is saved by Zero as kills a Level E vampire. Zero continued to suffer from his bloodlust and discovers the new student is actually the Pureblood he has been hunting, Shizuka Hio. Zero prevented Yuki from becoming a vampire when attempting to trade for Shizuka's blood that would have saved Zero. He fought his master's compulsion and shot Shizuka to stop her from biting Yuki. Zero swears he will never let Yuki become a vampire. Zero is stopped from killing Shizuka upon discovering his brother, Ichiru is still alive and Zero is prevented from drinking Shizuka's blood finding her already dead. The Vampire Council sends their men to kill Zero when he is blamed for Shizuka's death, but he is saved by Kaname.
Zero accompanies Yuki to investigate her past by looking through the hunter's files, the president of the Hunters taunts Zero by putting him in a room with blood soaked materials. He tells Zero to secure Kaname Kuran's blood as the blood of the Kurans is strong enough to act as an effective delay. During an attempt to find out why Kaname is hiding Yuki's past from her, Kaname convinces Zero to drink his blood. Ichiru returns to the school and later that night, Zero goes with Yuki for moral support. After Yuki collapses, Ichiru warns Zero to stay away from her. Zero is devastated and angry when he discovers that Yuki is actually a Pureblood vampire, Zero severed his friendship with her and likewise Yuki told him her vampire side "ate" her other side.
Return of Rido
Though Zero regained his sanity, the Hunters Association have him locked up, arrested by Yagari. Kaname revealed that his purpose in strengthening Zero was in order for him to protect Yuki and defeat Rido Kuran. Zero is shot by his twin brother and finally completed the Twin Hunter fragment after he is convinced to "eat" the dying Ichiru. With this act, along with being a twin vampire hunter of excellent lineage who has become a vampire, within Zero is the blood of three Purebloods, Yuki, Kaname and Shizuka, making Zero one of the most powerful characters in the series and saving him from becoming a Level E.
Zero develops powers are shown to be manipulation of vines within his body that are an extension of the Bloody Rose, which is capable of stopping Yuki's evolved Artemis Rod. Zero declared Yuki his enemy and attempted to keep her out of his fight with Rido. When Yuki struck the deadly blow to Rido's heart, Zero completed the destruction. Zero then turns the Bloody rose on Yuki and reveals his intention to kill all Pureblood vampires, including her. His memories of past Yuki fill him and he is unable to pull the trigger, yelling that it would have been ok if she had killed him instead.
Kaname interrupts the scene, but after a brief altercation he leaves Yuki and Zero alone to say goodbye. Because of Bloody Rose draining his blood, Zero once again feeds from Yuki, then kisses her, revealing to her that he loves her. Though he hugs her, Zero tells Yuki that the next time they meet, he'll kill her. As Yuki leaves the Academy, she sees Zero one last time, carrying Ichiru's body. The two lock eyes, then part ways.
After the one year time skip, Zero has been trying to find Kaname and hunting vampires on the hunter's execution list by night in an effort to sate his uncontrollable hunger.However, he has returned to the school, staying in an apartment owned by Kaien as he has been able to take the blood tablets for his thirst. He does not trust Kaname's intentions with regards to his peace negotiations, but decides to stay his hand because someone needs to control the vampires. He is to be made President of the Vampire Hunters Association in the future due to his strength and ability to intimidate others.
At school, he meets Kaito, another former student of Yagari and suspects Kaito intends to kill him, Kaito assures him that if Zero becomes violent, he will destroy him. Along with all the other hunters, Zero attends the negotiations.
Zero attends the party and becomes angry when he discovers that Kaito has brought Yori as bait, but Kaito mocks Zero's ability to protect Yori, assuring him its an opportunity. Though Zero sees Yuki at the party and she acknowledges him, he remains unresponsive. When Sara goes to "help" Yori, he grabs the Pureblood's arm away. Yuki interrupts them and asks Zero to let Sara go, touching his arm. Zero tells her not to touch him, she says she will let go if he lets go of Sara. Zero snatches his hand away and drags Yori off as well, revealing that he smelt fresh blood on Sara.
Later Aido asks Yori and Zero to accompany him to see Yuki privately, Zero escorts Yori and leaves saying he doesn't want to meet Yuki. Zero's arm starts reacting and he smells blood, finding the scene of the dead hunter. When Yuki says she intends to investigate, Zero harshly rebukes her for her naiveness. Zero confesses to Kaito about his hungers and desires to hunt vampires, Kaito offers to take Yuki's place to kill Zero should he turn dangerous. Neither Vampire Hunter trust the truth regarding the hunter's death.
Return of the Night Class
Zero and Kaito track Sara to the Dahlia Academy Girls School and Zero tries to confront her, but his overwhelming desire to hunt and her cutting words leave Sara ahead in the game. Zero tormented by his loss of humanity leads Kaito to advise him to take a few days off and to visit Ichiru's grave. Zero lies on Ichiru's grave and sees a vision of Ichiru urging him to accept himself as whole. Zero leaves the cemetery to encounter an injured Yuki. He takes her to a Hunter safe house to recuperate, where Yuki almost bites him.
Yuki and Zero meet up again after she is taken in by Kaien Cross. Zero appears bothered by Yuki's comings and goings, but he ends up helping her regardless. When the Night Class is developed Yuki thinks that Zero is totally against the idea of it and she also worries for Zero because he might be bloodlust for her blood and might turn into a Level E. When Zero kills a vampire prematurely and Yuki disagrees, he suggests she use her bracelet on him, but she declines saying it is only for when he falls to Level E.
Sara bothers Zero and Maria interrupts, the next day she finds Zero and talks to him about Ichiru inside him. Zero carries her back to her room with Yuki spying them. Yuki confronts Zero and tells him her concerns and she confesses she wants to talk to him like before. Zero thinks she has not had enough blood and tells her to drink his blood, he insists their interests are the same. Yuki agrees that its for her duty, not her desires that she drinks his blood. When she drinks, she spies a strange scene of a graveyard, with many tombstones, a vine wrapped around a coffin with a tree sitting on top, Kaname as the wolf and Yuki as a human also appear and tell Yuki that the fragments are protecting Zero's memories. Zero tells Yuki not to peek and she ceases to drink. Zero tells Kaien that if Kaname is killing off all of the Purebloods, there's no rush for Zero to stop him.