Katsuo Mizuno (Japanese: 水野カツオ, Mizuno Katsuo) is a freshmen at Seishun Academy and is also in the tennis club. He and Ryoma are friends, and as a member of the tennis club he is required to attend all of Seigaku's tennis tournaments to support the regulars. Albeit not very skilled, he aims to be able to be a regular one day but in the mean time he does not mind picking up balls during practice.