Masumi Souda is a supporting character in Red Data Girl who exists as a divine spirit. He was originally the older brother of Mayura Souda and Manatsu Souda, but he died at the age of six due to a heart defect while taking an afternoon nap. After his death, he became a spirit that only his siblings and the protagonist Izumiko Suzuhara are able to see. He shares an identical face with his younger brother Manatsu, which sometimes leads to confusion. Masumi has a playful and occasionally mischievous personality; he sometimes enjoys dressing in girl's clothing, much to Manatsu's embarrassment. Despite this lighthearted side, he is deeply protective of his siblings and does everything in his power to keep them safe when he is able to intervene. He wields formidable spiritual power and possesses the rare ability to alter his appearance at will. His role in the story is primarily as a guardian spirit who watches over Mayura and Manatsu at Hōjō Academy, and he occasionally interacts with Izumiko, offering subtle guidance or support from the spirit world. Masumi remains a static character throughout the narrative, as his existence as a deceased spirit prevents further personal development, but his loyalty and affection for his family remain central to his actions.