The exact nature of their relationship is unknown, but Akatsuki is the only one person who Ruka is vulnerable around, abandoning her self-important, bored front. Although Ruka does not appear to be in love with Akatsuki, he has been in love with her from a very young age and it is his love for Ruka that prevents him from having an undying loyalty to Kaname like Ichijo and Hanabusa.
BackgroundAkatsuki learned of Kaname biting Ruka when they first came to Cross Academy. He later told Kaname that although he doesn't necessarily mean he wanted Kaname to reciprocate Ruka's feelings, he would never forgive Kaname if he ends up causing her unhappiness. He then carried Ruka to her bed, kissed her forehead, and said they were both idiots. Kain is only concerned with Ruka's happiness from this point onwards.
Whilst Ruka, Aido and Kain were discussing their relationship with Kaname, Kain states: "Even though we both know that our loved one will never so much as look our way, we can't erase that small something within us that continues to hope in vain." Kain is referring to his own unrequited love for Ruka and Ruka's unrequited love for Kaname.
An entire chapter in the manga has been based on Akatsuki's romantic feelings for Ruka. After Ruka realizes why Kaname holds feelings for Yuki Kuran, her jealousy subsides. Ruka and Akatsuki are related in a certain way, as the two of them and Hanabusa are members of the “Aido faction.” Akatsuki and Ruka have known each other since they were very young, and Akatsuki, when he was young, told Ruka everything, including that Hanabusa had rebuffed Kaname. Akatsuki and Ruka are constantly together; he is the one
who pulls her away from Kaname during the Ichijo incident, comforts her when she realizes how little she understands Kaname, and dances with her at the ball during Volume 4 of the Vampire Knight manga.
Akatsuki and Ruka follow Kaname for a year after he and Yuki leave the Academy. They side with his plan to sacrifice himself to destroy the Pureblood vampires, except for Yuki, with the anti-vampire weapons. They block anyone who gets in Kaname's way, including Hanabusa and Yuki. Ruka sides with him with no question, but Akatsuki does so only to protect Ruka.However, Ruka is severely wounded by Kaname's anti-vampire sword during his fight with Zero. Akatsuki throws a fireball at Kaname's shoulder, saying that he will never forgive him for hurting Ruka. However, Ruka stops Akatsuki, saying that it was her own fault for acting recklessly. Kaname states that she has betrayed him and severs his ties with her. After he escapes, Yuki offers her blood to Ruka. But suddenly, Akatsuki sucks the blood from Yuki's wrist and gives it to Ruka through a kiss. With this action, he finally reveals his love for her.