Cha-Komon, whose real name is Aria, is a notable side character in the series. He is a cat who resides at Yumegaoka High School and is owned by the school's principal. The name Cha-Komon, which translates to Advisor Cha, originated from the main character Hikari Kohinata who, upon first meeting him, mistook him for the diving club's advisor. Most characters simply call him Cha because of the sound he makes.

In terms of appearance, Cha is a white-furred cat with brown ears and a brown and white striped tail. He has distinct blue eyes and a short muzzle. Two light-brown circles on top of his head resemble eyebrows, and his ears have a slightly drooping quality. Despite looking somewhat round, he is actually quite skinny.

Cha possesses a very friendly and fearless personality, showing no shyness around humans and approaching students whenever he feels inclined. He is known to climb onto people's bodies and ride around on their shoulders or heads, a habit he displayed from his first meeting with Hikari. A unique and notable aspect of his character is that he has a masochistic streak, reportedly preferring to be stepped on with feet rather than being petted with hands. He also has a surprising fear of water, which provides an interesting contrast to the series' diving-focused setting.

His primary motivation and key relationship center on another cat named Ohime, a stray adopted by the school principal. Cha is described as being in love with Ohime and acts as her dedicated knight and protector, making sure she is never in any danger. His role in the story is as a constant, free-roaming presence around the members of the Diving Club. He follows them wherever they go, even on car trips outside of school grounds, and finds ways to entertain himself by playing or bothering the club members. While not a club member himself, he is a beloved fixture in the club room and around the school.