Ranko Midorikawa is a supporting character in the anime Ace wo Nerae!. She is the ace player of Kaga High School, the rival institution to Nishi High School, where the protagonist Hiromi Oka is a student. Because of her school and her standing there, she is widely known by the nickname Kaga no O-Ran. She is a notably tall athlete at about 175 centimeters, with dark wavy hair worn off her forehead, and her physical build directly shapes her tennis identity: her signature weapon is a powerful serve that travels with the force of a bullet.

On the court, Ranko is portrayed as one of the strongest players in the story. She is the best player on her own team, and her overall level of ability is regarded as comparable to that of the celebrated Nishi player Reika Ryuzaki, nicknamed Madame Butterfly. Her competitive record is built around two sharp contrasts. Against Hiromi, she is consistently successful and never loses. Against Madame Butterfly, however, she remains winless, though she comes closer than most to breaking that pattern. In the singles final of one tournament, she pushes Reika to the brink of defeat before a sudden wrist injury forces her to withdraw from the match. In a later doubles competition, she and Reika face a team that includes Hiromi, and Ranko ends up on the losing side once Hiromi begins to read and return her serve effectively.

Ranko's role in the story deepens considerably when it is revealed that she is the half-sister of Jin Munakata, the demanding coach of Nishi High's tennis team, sharing a father but having a different mother. This revelation places her in a genuinely complicated emotional position, because her connection to Munakata works on several conflicting levels at once. They are family, they move in the same competitive tennis world, and he serves as the coach of the rival school whose team she leads as its star player. These overlapping bonds create tension and mixed feelings between them for much of the story, and a significant part of Ranko's personal arc involves working through that difficult relationship. Eventually, the two reconcile, allowing her to move past the resentment and confusion that the situation had created.

Ranko's relationship with Hiromi functions as a meaningful athletic rivalry. Her ability to defeat Hiromi time after time makes her both a genuine obstacle and a measuring stick for the protagonist's growth, since every encounter with Ranko forces Hiromi to raise her own game. Her rivalry with Reika provides a different kind of motivation. Despite being one of the few players capable of threatening the top-level competition around her, Ranko is repeatedly denied victory against Reika, and her near miss in the tournament final highlights both her enormous potential and the frustration that comes with falling just short. She is defined by the pride and intensity expected of a school ace, but also by the personal struggles beneath that confident exterior, including the burden of her hidden family connection to the coach of her school's biggest rival. Her story is one of a fierce competitor learning to separate her tennis ambitions from the complicated loyalties and resentments tied to her family history.