Mamiko Oumae is a side character in the Sound! Euphonium series and the older sister of the protagonist, Kumiko Oumae. She is five years older than Kumiko and, during the main events of the story, is a college student. Mamiko was once a talented trombone player and played a significant role in sparking Kumiko’s interest in brass instruments by teaching her the basics, such as how to use a mouthpiece. However, during her own middle school years, she gave up playing the trombone in order to focus on preparing for her college entrance exams, a decision that deeply affected her emotionally.

Mamiko has an upfront and often cheerful personality, though she is quick to anger and can be blunt, especially with her younger sister. Beneath this sharp exterior, she hides a warm and caring side, and she genuinely supports Kumiko even when their relationship becomes strained. She is thoughtful but capable of jealousy and can sometimes be blind to others’ feelings. As a high schooler she was happy and active in band, but the pressure to be the “perfect” child in her parents’ eyes led her to abandon music, a source of inner conflict that fuels her rebellious streak in her later years. Her motivations revolve around finding her own identity separate from family expectations, and she eventually works toward reconnecting with the joy of music.

In the story, Mamiko functions as both a source of tension and a supportive figure for Kumiko. Their relationship is initially marked by conflict, particularly over Kumiko’s continued involvement in music, which reminds Mamiko of her own sacrificed passion. Over time, Mamiko evolves from a critical, distant sibling into a genuine mentor, offering advice and encouragement. Her key relationship is with Kumiko, but she also interacts with their parents, whose high expectations contributed to her retreat from music.

Notable abilities include her skill as a trombone player, her deep musical knowledge, and her meticulous, organized nature. She is physically described as a pretty young woman with long, wavy brown hair, often styled, and a preference for dramatic makeup and flashy nails. Despite her talents, she is notably bad at cooking, with her attempts often ending in disaster. Mamiko’s development throughout the series centers on her journey toward self-acceptance and rediscovering her love for music, which parallels and enriches Kumiko’s own growth.