The Promise is the fourth chapter in the Vampire Knight manga:
Recap of images of Zero biting Yuki's neck. Zero wipes his mouth clean and Yuki clutches her neck, staring in horror at Zero as she realizes he is a vampire. Kaname stops amid his conversation with Kaien and looks around, smelling blood, he rushes out the door.
Zero says Yuki's name and takes a step closer, Yuki trembles, staring at Zero still. From down the stairs, Kaname says Yuki's name and jolts her out of her shock, Kaname gets angry as he takes in the scene of a bloody Yuki and Zero. He pulls Yuki behind him protectively asking Zero if he's reduced to being a beast. Yuki has a sudden memory flash of her first encounter with Kaname killing the other vampire. She jumps out in front of Zero, afraid Kaname will kill Zero. The effort drains Yuki and she slumps into Zero's arms. Kaname takes Yuki out of Zero's arms blaming him for draining her too much and asks if her blood was that delicious while glaring at Zero. Yuki begins to wake and says Zero's name. She asks Kaname to put her down and asks for Zero again, but she cannot struggle. Kaien arrives.
Kaname takes Yuki to the infirmary, Yuki is now sitting up and protesting that she is fine. He checks her puncture noting that the blood has stopped but the wound is deep. He asks if it hurt noticing that she is now crying. She is surprised by her tears and shakes her head. She thinks to herself that she is confused, but now believes she hurt Zero. Kaien knocks at the door and advises Kaname to return to class to settle the other vampires.
Kaien tells Yuki he took Zero to his room and confesses that he couldn't tell her the truth before. Yuki protests that she had no idea that Zero was a vampire, Kaien tells her what happened to Zero, she clutches her neck and Kaien assures her she'll be okay because Zero isn't a Pureblood. Yuki only thinks about how Zero suffered for four years all by himself. Zero lies alone in his room.
In class, Sayori observes to Yuki that Zero is missing. Yuki tells her that Zero isn't well. Yuki thinks about how much Zero loathes vampires and that he must have wanted to kill himself too. Zero is in his room, packing his stuff, a photo of Yuki and Zero falls to the floor at his feet. Zero sits on the bed holding the Bloody Rose and arms it. He recalls Kaien telling him it would only harm vampires. He holds the gun up to his head and then Yuki is at the door. She yells his name and jumps on him grabbing the gun, they fall onto the bed. She yells at him, asking him what he was trying to do, he denies doing anything, she calls him a liar saying the safety was unlocked. He looks up at her, asking what she was doing there, he grips her wrist holding the gun and flips them over, now holding her beneath him. He rips the bandage off her neck and asks if she heard the sound of her blood being sucked by him and tells her she can't act as if nothing had changed and that she can no longer interfere. He grabs his bag and leaves.
Yuki stays lying on the bed, her and touches the photo and she sits up to look at it, remember the day it was taken. She remembers her decision to always be by his side, she grabs the Bloody Rose and runs after Zero. She follows Zero asking him where he was going, but finds herself becoming dizzy and falls against the wall. Zero tries to take the Bloody Rose from her hand telling her she'll hurt her shoulder if she doesn't stop. Yuki refuses to let him go like this and she struggles to hold up the gun. Zero pulls it to his chin, saying that the next human he targets he may kill and tells her to shoot him. He tells her she is afraid of him. He tells her how to aim and that it is not a crime to kill a vampire.
Yuki refuses, she drops the gun and grabs Zero from behind, hugging him tightly, she knows the pain he is hiding. She tells him she doesn't understand what he went through in the last four years, but she wasn't afraid of him. She promises to stop him, if the time comes. She promises to be his ally. While Zero looks sad.
Kaname sits in his office opening letters, he pauses for a moment as he cuts himself and then stabs his letter opener into the image of Zero in their photo.