Sumire Takagawa is a classmate and a close friend of Takagi, present since the first volume of the manga. She rarely takes the spotlight and speaks little. Her full name is revealed later, with some uncertainty regarding the pronunciation of her family name. She was once credited in the anime as "the girl with the ponytail" before her name was known.

Recognizable by her ponytail and gentle, cute appearance, she often sits at the front right diagonal from Nishikata in class. Her friendly, kind, and calm personality led to a strong friendship with Takagi after they were seated next to each other on the first day of school. They bonded over a shared enjoyment of walking and quiet time, and they continue to spend time together during and after school.

She demonstrates a special skill in performing tarot card fortune telling, particularly about love, which she sometimes does for classmates. Thoughtful in relationships, she gives Valentine's chocolate to a boy named Sakurai. Her friendship with Takagi endures for years, as shown in a future-set story where Takagi plans a trip with her.

In the anime adaptation, her seat and fortune-telling ability are special features created for the series. She appears in manga chapters including "Beach Trip" and "Awaited Person," where Takagi calls her "Sumire-chan" and Nishikata addresses her as "Takagawa-san." Although not directly appearing in the spin-off "Teasing Master (Former) Takagi-san," her name is mentioned, confirming her ongoing importance to Takagi.