Reiko is a very slim woman with neck-length dark brown hair and light brown eyes. She is usually seen wearing a black women suit with a open blouse showing her pearl necklace, red lip stick and stud earrings.
Little of Reiko's personality is known, but she occasionally scolds Taichi Mashima (Reiko's son), if he does not win a competition, in which Chihaya Ayase calls her "Mrs. Pressure" by the way she treats Taichi. Yusei Nishida thinks she is scary. She is a believer of the phrase "Rather be a big fish in a little pond, than a small fish in a big pond." By this quote she also raised her children, especially Taichi.
Because she does not like for Taichi to lose, she often asked him to quit karuta. However, once he did quit his karuta club she has shown the first time also regret for her pressure.
Being perceived as a cold-hearted women, she still shows passion and tears as she talks about her husband, as well as when she found out that Taichi has won his preliminary matches for the Meijin title.
Reiko appears from time to time as the strict mother she is. When Taichi Mashima and his friends from the karuta club come to visit, she pays close attention to them doing nothing unruly.
At the beginning of the story she constantly pressures Taichi to quit karuta as she believes it to interfere with his studies. But Taichi managed to convince her to let him play as long as he managed to get the top score in every subjects.
However, at some point Taichi quit his karuta club. Reiko noticed and showed the first time regrets for pressuring Taichi to quit playing karuta.
As Taichi returned to playing matches and winning to go for the Meijin title, Reiko is so overwhelmed that she immediately talks to Rieko Oe to make a new men's hakama for him. However, as the time came (Chapter 192) to pick out the cloth for the Hakama, Reiko gets a call that her daughter is sick. She rushes off with the note that Taichi will be fine on his own to pick out the proper clothing for his upcoming matches.
Reiko met Mr. Mashima during High School times and she was with him together on duty as the Public Morals patrol. He never knew how to act around her but must have drawn her attention enough that she gave her consent to marry him.
- The name Reiko means "beautiful, lovely" (麗) (rei) and "child" (子) (ko).
- Reiko's surname Mashima means "real, genuine" (真) (ma) and "island" (島) (shima).