Shizuko is a fair-skinned young girl of average height. She has short black hair and oval glasses that obscure her eyes. She is usually seen wearing her school uniform. Also, she isn't very good at physical activities as she claimed herself.
Shizuko is a sensible and intelligent girl who often spends lunchtime with Misaki Ayuzawa and Sakura Hanazono. Shizuko is good at math, a member of the flower arranging club, but isn't very good at physical activities. She is also a calm, respectful, rational, mature and modest girl. Shizuko is usually quiet and passive and doesn't like to get herself into problems. She doesn't like it when people assault glasses in general. Shizuko often has angry outbursts at Sakura, reprimanding her for her tendency to fall in love so quickly with the wrong guys. She also gets angry for Misaki for being impatient and making a scene. When she gets angry she can be very scary. Shizuko is also strict and finds meeting strangers, attending concerts and going on trips a nuisance.
Not much is known about her past except Shizuko has an older brother in university and a younger sister in junior high school. According to Sakura Hanazono, the three of them look very alike because they wear the same glasses as Shizuko.
Shizuko first appears alongside Sakura Hanazono in the school, requesting that Misaki Ayuzawa help them. Taking her to a sandbag, the girls reveal that the boxing club left it there and it is blocking the entrance to their classroom. As Misaki moves the sandbag herself, Shizuko is left without words at what she sees. Once Takumi Usui is spotted watching them, Shizuko listens to Sakura rave about how great he is and note that he is a popular person who he is very popular. Shizkuo notes that though he rejects all girls, it does not lessen the number of confessions.
Sakura and Shizuko give Misaki a call in which they ask her to stop a fight between two Seika High School students and three students from the prestigious Miyabigaoka High School. In Episode 10, Sakura invites Misaki and Shizuko to a tea party that the band hosts next week at a café where the Idiot Trio are working part-time as waiters. In episode 11, Sakura, Shizuko and Misaki decide to follow Usui to have a peek at his private life and confirm rumors that he is rich. However, Usui knows he's being followed, so he leads the three on a wild goose chase around town.
During episode 12, when the sports festival is held, at the obstacle race, where its first prize is a kiss from an unwilling and horrified Sakura, Misaki is leading until one of the boys pushes her into the pool to disqualify her but Takumi saves her from falling. Takumi wins the race but decides to give the prize to Misaki, who came runner-up. Later, Misaki enters the costume race on behalf of Shizuko.
In episode 15 Sakura, Shizuko and other girls are open a cafeteria for the school festival. In episode 22, Sakura along with the other students goes at a Buddhist temple for five days and four nights. In episode 25, Sakura invites Misaki and Shizuko to Yumesaki's Culture Festival to attend UxMishi's concert.
Misaki is one of Shizuko's good friends. Along with Sakura, they are often seen doing things together. As revealed in episode 11, she is very good with computers, but is evidently an average student, as she is only in class 2-3 with Sakura. According to Sakura, she has a brother and sister in university and junior high, and that all three of them look alike.
- The name Shizuko means "quiet" (静) (shizu) and "child" (子) (ko), which fits her personality.
- Shizuko's surname Kaga means "increase," "step up" (加) (ka) and "congratulation" (賀) (ga).
- According to the character profile in the manga:
|Misaki Ayuzawa • Sōtarō Kanō • Shōichirō Yukimura • Takashi Hagimoto • Kenji Sawa • Kiyomasa Katada • Hayase Kōta • Yūta Utsumi • Bunta Kosugi|
|Naoya Shirokawa • Ryūnosuke Kurosaki • Ikuto Sarashina|
|Takumi Usui • Hinata Shintani • Shizuko Kaga • Sakura Hanazono • Suzuna Ayuzawa • Yumeyama Brothers • Takezawa • Gonda • Masaru Gōda • Hideki Suzuki|