Masumi Sōda is a deceased member of the Sōda family and the late brother of Mayura Sōda and Manatsu Sōda. He died at the age of six due to a heart defect while taking an afternoon nap. After his death, he became a divine spirit, no longer existing as a human soul but as a spiritual entity formed from the wish of the three Sōda triplets. He is bound to the region of Togakushi and can only be seen by his siblings and by Izumiko Suzuhara, the story’s protagonist. Despite his young age at death, Masumi has existed in spirit form for years and displays a personality that blends childish playfulness with a deep, unwavering protectiveness toward his family. He enjoys dressing in girls’ clothing, which often embarrasses his brother Manatsu, and he approaches situations with a calm, occasionally mischievous demeanor.

Masumi’s primary motivation is the safety and well-being of Mayura and Manatsu. He acts as their guardian spirit and is summoned by them when they need spiritual protection or aid. He takes this role seriously, intervening in dangerous encounters without hesitation. In combat, Masumi demonstrates formidable spiritual power, including the ability to devour other spirits. He can also alter his appearance at will, allowing him to take on forms that suit the moment, such as disguising himself as Mayura. His appearance closely resembles that of Manatsu, though his skin is noticeably paler. His voice and mannerisms sometimes give away the disguise, but his shape‑shifting ability is a key tool.

Masumi’s role in the story is that of a supporting, supernatural ally. He is not a central protagonist but becomes crucial during conflicts that threaten his siblings or the balance of the spiritual world. His relationship with Mayura and Manatsu is the core of his existence; they communicate with him often and rely on his strength. He also develops a limited connection with Izumiko, who can perceive him, and he assists in events that involve her. Masumi does not undergo significant personal development because he is already a static, transcendent being, but his presence influences the growth of his siblings, particularly Manatsu, who shares the same heart condition and grapples with his own mortality.

In terms of abilities, Masumi wields immense spiritual energy. He can manifest physically when summoned, fight off hostile spirits and spiritual attacks, and change his form. His power is such that he can overwhelm other powerful spirits and even consume them. He also has a degree of independence, appearing and acting on his own when his siblings are in peril. Despite his supernatural nature, his personality remains grounded in the bonds he had in life, making him both a formidable force and a deeply familial presence in the narrative.