Yamato Agari's best and oldest friend. Recently due to certain circumstances became the newest master of Jealousy.
Kodaira Rune is a smart boy, and Yamato's best friend since they were only 10 years old. In a past life, Rune was actually part of the nobility, and was actually a woman named Lady Gekko, who was romatically involved with Yamato's past self. This is one of the main causes of the future events leading to him becoming Jealousy's new master (taking the place of Iruma Tomomitsu, who was killed by Jealousy himself). Jealousy convinced him to become his master by arousing the hidden jealousy inside him revealing that, despite his gender change, he is bitterly jealous of Sayama because she (possibly unknowingly) is the object of Yamato's affections.
While it is not directly said, it is heavily and very obviously implied that he is still in love Yamato, even in the current era. On a lighter note, it is said in an extra that his favorite foods are sweets with whipped cream.