The exact opposite of most of the characters in the show: loud, rough-around-the-edges, she loves doing what she can to push other peoples' buttons. She enjoys mercilessly teasing or being mischievous, conducting most of her antics around Fukuzawa Yumi.

Despite being a non-conformist in the ranks of the regular girls in Lillian Jogakuen, Sei is one of the wisest and caring characters on the show. Though seeming to have a playful and laid-back attitude, she has experienced many tragedies in her life, especially in her second year.

Sei is known for doing things at her own pace, and dislikes being pressured into making decisions and taking action. She took Toudou Shimako as her soeur while in her third year. Her relationship with the first year is seen to one of mutual understanding and selective support. Sei however has an especially close relationship with Yumi. Sei becomes a source of comfort, and emotional outlet and close friend and mentor for Yumi.

She is often noticed by others for her unconventional looks; many strangers have assumed that she is a model. Sei is not purely Japanese, and has Anglo features, as seen by her hair and eye colour as well as her Caucasian facial bone structure.

After leaving Lillian, Sei studies English and American Literature. Part of this could result from her bibliophile behaviours she picked up during her second year in Lillian when she immersed herself in reading as a form of escapism and trying to understand her conflicting emotions during that time.

She drives a yellow Beetle and is a terrible driver.