Marcia O'Brien, sometimes spelled Marcia O'Brian, is a supporting character in the anime Baby Steps, appearing primarily in the second season. She is an American tennis player and a member of the Florida Tennis Academy, also referred to as IMG Academy. She is the sister of Alex O'Brien, who becomes Eiichiro Maruo's roommate during his training in Florida. Marcia is the first opponent Eiichiro faces upon arriving in the United States, making her an early benchmark for the level of competition overseas.

Physically, Marcia is a pretty girl with long blond hair and a mature appearance, which surprises Eiichiro when he learns her actual age. Her background as a student at a prestigious tennis academy indicates a serious commitment to the sport, though her specific motivations and personal goals are not deeply explored in the series. She appears confident and composed on the court, befitting a player trained in a competitive international environment.

In terms of role, Marcia serves as a brief but significant test for Eiichiro, allowing him to measure his skills against an American player and gain firsthand experience of a different playing style. Her match against him helps highlight the differences between Japanese and American tennis approaches. Her relationship with her brother Alex is noted, but beyond that, she does not form lasting connections with the main cast. Her character development is minimal, as her appearance is limited to a small arc during Eiichiro's overseas training. Her notable abilities include solid tennis fundamentals and the composure expected of a Florida Tennis Academy trainee, though no signature techniques or extraordinary talents are emphasized. Overall, Marcia O'Brien is a minor figure whose primary narrative function is to introduce Eiichiro to the international tennis scene.