Ruka once offered her blood to Kaname while Yuki was sleeping as a child. She knew that Kaname was protecting Yuki for a reason and that if he bit her he would later regret it. They then left the room and Kaname bit Ruka, like she had wanted. Kaname did it to show Yuki what he truly was, hoping it would keep her away from him for a while. Ruka knew that there would be repercussions but believed that her actions were right.
Ruka's one of the many collateral-effects Kaname made following his plans still, she somewhat obliges and stays by the Pureblood's side, knowing she has nearly no chance of more than a friendship. Ruka seems to have given up on trying to get Kaname to love her and has accepted that he will not. She locks herself in her room after she hears that Kaname has declared that Yuki is now his lover, but she soon accepts it.
Ruka follows Kaname for one year, and she and Akatsuki side with him with his plans. They block all of those who try to hinder his plans, including Hanabusa and Yuki. However, during the fight between Kaname and Zero, Ruka steps in between and shields Zero from Kaname, getting injured by his anti-vampire sword. Kaname declares this as an act of betrayal and that it is over between them. But even with their ties severed, Ruka protects him by joining with the Hunters at the Academy and fighting against the Pureblood vampires who wish to capture him.