The eldest son of the Sumimuras. Although he is the oldest of the three brothers, he is not the heir. He is 21, though he looks much older. Masamori is a leader of a setion in the Urakai organization. He returns home few times and stays for many days. He is a very skilled Kekkaishi; using techniques difficult to master. Masamori and Yoshimori have a strange relationship, Yoshimori being clearly jealous of the way Masamori has control over difficult techniques and the way Tokine acts around him. However, Masamori is often seen looking at the palm of his right hand, where the sign of the "legitimate successor" is placed; this suggests that he is jealous of Yoshimori, although he doesn't know so. Yoshimori often wonders why his older brother wasn't chosen to be the legitimate successor.