Little Mugi and little Sumire

Sumire alongside Tsumugi as little kids.

Originally, Sumire's family originates from Austria. At some point before Sumire was born, her family moved to Japan to serve the wealthy and respectable Kotobuki family. After the family moved to Japan, it took on the surname "Saitō".

Sumire grew up in the household of the Kotobuki family, serving as a maid. The only daughter of the Kotobuki family, Tsumugi Kotobuki, quickly became a person Sumire loved as if they really would be related to each other and the two forged a strong bond. Since Tsumugi was highly treasured by her family and received a special education, she grew up isolated from other people of her age and spend much of her time with Sumire who often helped her finding out about the lives of "ordinary" girls, for example by bootlegging manga for her.

Sumire convincing Tsumugi

Sumire convincing Tsumugi to do as their families told them to.

Around the time Sumire turned twelve, her father clarified the relationship between the Kotobuki family and their own, something Sumire was oblivious of her whole life before. At first she was daunted by the readjustment she has to follow in the future but after she heard that Tsumugi plans to disregard the plans her family have with her as well, she changed her mind and pushed Tsumugi to follow them until she attends university. She then started to refer to her as "Tsumugi-ojōsama" (which approximately means "Lady") instead of "Onēchan" (which refers to an older sister) in public.

After graduating from Junior High School, she applied to the Sakuragaoka High School and was accepted. Since Tsumugi already graduated and the room of the Light Music Club was still equipped with the tea sets she brought with her three years ago, Sumire was tasked to take them with her.

K-ON! High School, Chapter 1Edit

Sumire trying to remove the tea set

Sumire being caught red-handed by the Light Music Club members.

Right after starting her high school career, Sumire began to carry Tsumugi's order into execution and tried to remove the tea sets from the Light Music Club's room. During the process, she was interrupted by the club's members Azusa Nakano, Ui Hirasawa and Jun Suzuki. Due to her european inheritance, they mistook her for an foreigner and tried to talk to her in English.