Rune (sometimes spelled as "Lune") is Yamato's best friend and was there the day that Yamato found Ultimo in the antique store.
His douji is Jealous.
During the past life he was apparently a girl, and seemingly in love with Yamato, who may have returned her love, but in the present he is a boy. After Jealous revealed Rune's memories from his past life to him, Rune and Jealous later attacked Yamato at school. Rune captured the girls and seemed upset that Yamato paid so much attention to Sayama. Recently, in Chapter 10, the story of when he was a girl in his past life was revealed.