Sanae Sumida, daughter of the Sumida financial group and first ex-wife of Genichirou Miyanomori, is mother to his eldest son, Tadashi. Her marriage served primarily to produce a Miyanomori heir; she views Tadashi as a tool to secure the family's leadership.

Strict and prideful, Sanae adheres to traditional views of female subservience. She moves with aloof, unapproachable precision, displaying arrogance towards those she deems beneath her, particularly servants whom she considers "low and filthy." She openly criticizes Genichirou's festivals as deceitful and fiercely defends the Sumida lineage's prestige against what she perceives as the Miyanomori's lesser history, confronting Genichirou about his self-serving marital motives.

Her appearance reflects tradition: long black hair swept into a tight bun, exclusively worn kimonos or yukatas, with kimono collecting noted as a personal hobby.

Sanae's relationship with Tadashi is strained and transactional. She relentlessly pressures him to outperform his half-brothers for the heir position, reinforcing his role as a status instrument. Their interactions often leave Tadashi in foul moods that hamper his work. When Tadashi visits with a birthday gift, she dismisses him immediately upon receiving it, highlighting their emotional distance. Despite this, she covertly collaborates with Genichirou's estranged son Mamoru, granting him access to family events to undermine the Miyanomori for her own agenda.

Her role consistently centers on upholding the Sumida legacy through Tadashi while perpetuating friction within the Miyanomori household.