A K-ON! High School character, she debuted after the April 2011 manga relaunch. An incoming freshman and light music club member at Sakuragaoka High School, she joins when she is caught trying to move Kotobuki Tsumugi's tea sets out of the club room on her orders.
Eventually, the nature of their relationship is revealed. The Saitou family has been serving the Kotobuki household for years, with Sumire serving as a maid and growing up with Mugi, who she regarded as her older sister.
