It has happened with him a lot in the novels and is seen in the anime. First he becomes his servant the day he finds her asking for help in one of the subways of the city since he was about to die because of Episode, Guillotine Cutter and Dramaturgie, three specialists who fought against her, in that fierce battle his legs, arms and heart are torn away. That weakens Kiss-Shot too much. That's when Araragi Koyomi, a boy who was going through an existential moment in his life, hears the vampire's gips and comes closer when he finds her, he suffers panic and after reflecting he thinks he gives his blood to Kiss- Shot would die but it wasn't like that.
Koyomi simply becomes a vampire and Kiss-Shot threatens him to take charge of gathering his members, Araragi fights and wins the battles but at the moment the vampire returns to its full form, it kills Dramaturgie, Koyomi sees the scene and he begins to worry about his vampire instincts and fights Kiss-Shot to find a way to regain his humanity. Taking advantage of Oshino Meme councils, he manages to defeat Kiss-Shot by draining all his vital energy but not the killer only creates a link which the vampire as a rarity lost part of his vampiricity and Koyomi regained his humanity a little leaving both in a threshold between races. From there Kiss-Shot now named Shinobu Oshino becomes a servant of Araragi.