Akatsuki & Ruka

Relationship outline
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.

Background
At one point, Akatsuki saw Kaname biting Ruka and said although he doesn't necessarily mean he wanted Kaname to reciprocate Ruka's feelings, he would never forgive Kaname if, in the future, he end up causing her unhappiness. He then carried Ruka to her bed, kissed her forehead, and said they were both idiots.

"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." ~ Akatsuki Kain Chapter 48

Plot Summary
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 Cross, her jealousy subsides. Ruka and Akatsuki are related in a certian 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 "Vampire Knight."