Marcia is a pretty and mature looking girl, which surprises Eiichiro when her age is revealed. She has long blond hair and wears a ponytail during tennis matches.
She is very strong-willed and has a slightly stubborn nature, which is revealed when she refuses to accept losing. She easily becomes friends with Eiichiro and appears to admire her brother Alex greatly.
According to Eiichiro, Marcia's shot stretch really far which is pretty amazing for someone with her slender body.
During their practice, Marcia thought of Eiichiro as a machine who was able to return every ball. Later, when he said that he was aiming for high school level nationals only, she called him a small man. She helped Eiichiro adjust to the Academy later on, but still refused to accept him as a good player, until Eiichiro defeated her brother.
When Eiichiro is back in America, it is hinted that she might have started to have an affection for him, as she is blushing when she talks to him or when she watches him play. She also invites him to do things with her more often than before. 
Alex is Marcia's older brother and she cares a lot about him. She looks up to him as he is a good player. Marcia wants to become just as good as him someday.