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 a perfectionist. He earned his nickname "Ei-chan" because he keeps getting straight As in exams. Although an honor student, he lacks in exercise, as a result, he ended up joining the Southern Tennis Club(often abbreviated as STC).

At the start of the series, he starts off as a naive beginner who has trouble in trying to catch up and learn the basics.

He has romantic feelings for Takasaki Natsu or Natchan as he calls her affectionately, his fellow club and school mate.