Tsukushi Tsutsukakushi

One of the heroines of the story. She refers to herself as "I." She is Tsukiko's older sister and a third-year high school student who is the captain of the track and field club. With her long hair styled in a ponytail, she is a stunning beauty with an excellent figure and athletic prowess, sharing the same bluish eyes as her sister. Yōto describes her as "lion-like."

Her personality is extremely strict, and because she rarely shows her emotions, she is feared and nicknamed "The Steel King" by those around her. However, she also has a kind side, as she cares about Yōto, who is isolated in the club and called the "Perverted Prince," and mediates between Yōto and Mai when they quarrel. After receiving the "true feelings" from Tsukiko, her expressions became more varied, improving her relationships with others, and Yōto started calling her "Steel-san." She is also very trusting and has a strong tendency to jump to conclusions. She loves "exciting things" like fantasy and imaginative stories.

She is a big fan of board games and has created several original games. She is also skilled with her hands and has made a statue of the Laughing Cat in the past.

She has an extreme sister complex and, despite being poor at English and foreign words, wishes to attend the Massachusetts Institute of Technology in Massachusetts, where same-sex marriage is recognized, to marry her sister. She initially saw Yōto and his younger brother as enemies for bothering her sister, but after an incident at the Tsutsukakushi household, she began to have feelings for Yōto's younger brother as well and is considering moving to the Arabian Peninsula, where polygamy is recognized.

She specializes in javelin throw and has won the national high school championship three times in a row. She is also well-versed in martial arts, but her academic performance is poor. She has received sports scholarship offers from prestigious universities like Waseda University, but she has turned them all down the next day, disappointing Tsukiko. Yōto was also disheartened when he heard her plan to apply to the University of Tokyo as a backup.

In her childhood, she was diligent but lacked consideration for others. Her current personality was formed after she vowed to her mother, Tsukasa, that she would not study until she learned various things from her mother.