Kaoru is in Gorou's class in elementary school and eventually begins to play little league baseball on the Mifune Dolphins Little League after an argument with Gorou where she sees his passion for the game. In Junior High School she switches to softball and becomes captain of the Mifune East Junior High team.

Initially she plans to follow Gorou, whom she has a crush on, to Kaido High School. However, she changes her mind after she discovers Kaido does not have a softball team and after Gorou tells her to follow her love of softball.
After high school, she attended Kyowa University. During the winter break of her freshman year, she and Gorou finally became a couple.
At the end of the series, Shimizu marries Goro and they have two kids.