Shūichi Natsume is Akira Natsume's older brother and Kaori Munakata's boyfriend. He presents as a tall young man with short black hair, a marked contrast to his childhood appearance where he was chubby and wore glasses.

Shūichi and Kaori encountered Akira after school one day. Kaori introduced herself to Akira and requested his help repairing her phone before taking a selfie with him; afterward, she and Shūichi departed. Later, at home, Shūichi shared a meal Akira prepared. When Akira asked how Shūichi met Kaori, Shūichi explained they connected through shared academic interests in university lab research focused on androids, correcting Akira's mistaken assumption about their work involving sex dolls.

During this conversation, Akira inquired about Shūichi's significant physical transformation since childhood. Shūichi attributed the change to personal effort and advised Akira to find a purpose and take initiative to instigate meaningful change in his own life. His name, Shūichi (柊一), combines "holly osmanthus" (柊, shu) and "one" (一, ichi), while his surname Natsume (夏目) signifies "summer" (夏, natsu) and "eye" or "ogle" (目, me).