A fifteen year old prodigy, whose most prominent feature is his hair, often called "rooster hair" by his colleagues.

Maruo Eiichirou is a diligent student and somewhat of a perfectionist. He earned his nickname "Ei-chan" because he keeps getting straight A's on his exams. Although an honor student, he lacks in exercise, and as a result, ends up joining the Southern Tennis Club (often abbreviated as STC).

It is revealed later on in the manga that his mother was the one who suggested joining a tennis club.
He starts off as a naive beginner who has trouble in trying to catch up and learn the basics.

However, he soon paves his way against tougher opponents such as seeded players in prefecture tournaments. After learning he needs to improve for the upcoming hurdles he will have to face, he takes up the suggestion on flying to Florida in an attempt to improve his overall performance in tennis. Although he loses his practice matches in the first few days, he eventually improves and even beats Alex, a professional player who is included in the world's top 1000.
He has romantic feelings for Takasaki Natsu or Natchan, as he calls her affectionately, his fellow club and schoolmate.

Later on in the manga, they start going out.