Ruka Souen is a female vampire popular with the Day Class students and one of those who are the most faithful and protective of Kaname. She is a childhood friend to Kaname, Ichijo, Aido, and Kain.
Ruka appears to dislike Yuki because Kaname loves her while Ruka's own strong feelings for him remain unreciprocated. It was expressed in a flashback that even before she met Kaname, she had always had feelings for him to the extent of wanting to give him her blood. However, the only time he ever drank her blood was to protect Yuki from his urge to bite her, and he has since politely refused to drink it. Ruka is upset when Kaname chooses Yuki to be his lover, but finally accepts her when she discovers that Yuki is Kaname's pureblood sister, even having Aido give Yuki a pair of shoes to wear. Recognizing the foolishness of her obsessive feelings for Kaname, Ruka shows that while she remains as decicated as ever to him, she is able to do so calmly and with a mature level-headed attitude.
During Rido's attack on Cross Academy, Ruka works with Kain to defend girls in the Day Class.
an extended description!