Ruka Souen (早園 瑠佳, Sōen Ruka?) is a female noble vampire who provides a stark contrast to Yuki Cross. She is 169 cm tall. Ruka has the ability to weave illusions, thus she can trick enemies into killing each other or themselves. She has been in love with Kaname Kuran since she was a child and thus remains close to him as a faithful friend. However, she ironically remains unaware of Kain's unrequited love for her. She is also long-time friends to her cousins Aido and Kain. She has an intense jealousy towards any humans that raise Kaname's interest. Her character becomes more jealous and disheartened towards the end of the first arc after she learns that Yuki is engaged to Kaname. Despite all this, she continues to maintain an unrequited love for Kaname. Like the rest of the Night Class, she works with Kain and Aido to defend girls in the Day Class during the attack on Cross Academy from hostile vampires.

After Yuki's transformation into her former vampire self, Ruka becomes closer to Yuki, such as giving her etiquette lessons, though she is still intensely jealous of her. While Ruka is kind to Yuki and other girls, she avoids them when she can. Yuki eventually confronts Kaname about her, to which Kaname tells Yuki that it will be a long time before Ruka can forgive him for once drinking her blood, consequently making Ruka believe he loved her but never truly repaying her for the act. In Chapter 81, Ruka helps Kaname get away from Yuki by creating an illusion of Zero killing her, and later apologizes to Kaname about letting Yuki see something horrible. However, in Chapter 83, Ruka protects Zero from Kaname and is severely wounded by his anti-vampire sword. Kaname considers this an act of betrayal and tells her that it is over between them. After Kaname escapes, Ruka finally realizes Akatsuki's love for her when he kisses her, giving Yuki's blood to heal her wounds.