Mirei Shizuoka is a second year student at Umineko Shōgyō High School and a member of the school’s swimming club.
Mirei has large breasts and shoulder length brown hair. Because of her appearance, she is very popular amongst the male students. Her breasts get a lot of attention, from (wardrobe) accidents, to people fondling them (Amuro, Makio), to people being envious of them (Maaya, Karena Nanjou). In the manga, over time a couple of (female) club members commented on her breasts getting bigger.
She is very self-conscious and shy and does not like to be the center of attention or people staring at her body. But secretly she wears sexy underwear, likes objects she thinks are shaped like a penis and despite hating people staring at her, the idea that people get excited by doing so excites her. She has a crush on Kaname Okiura.
Mirei’s parents run the ‘Shizu Group’, a corporation, which is into railroads, department stores and amusement parks and was founded by her grandfather. Kaname thinks Mirei’s secret, more perverted, personality is a family trait, because her parents are into erotic cosplaying.
One of the first incidents that make Kaname get to know the real Mirei is when, shortly after Amuro joining the swimming club, due to the wind blowing up her skirt, Kaname notices her sexy underwear.
When the club has its first training camp since Amuro joining, Mirei, because of her self-consciousness, decides not to join the other girls when they take a bath. Instead she takes a bath when the other girls are finished, not realising the boys had not taken one either yet. She manages to escape the bath almost unnoticed. (Kaname notices someone, but does not recognise her.) In her hurry to get out, she accidentally forgets her panties, which Kaname finds, realising whose they are.
During a visit to her home Kaname and Amuro discover the bra matching the pair of panties that Kaname found, proving they are indeed hers. They also learn that her parents are into erotic cosplaying. During a later visit to the Shizuoka family second home in the mountains, he noticed this again.
At the prefectural tournament Karena Nanjou, the captain of the rivalling Shinomiya High School swimming team brags to Momoko about the accomplishments of her and her team and about the size of her breasts. Amuro joins in, fondles Karena’s and Mirei’s breasts, claiming Mirei’s are bigger.
When, on their way to the Kantou tournament, the club stops at an inn to spend the night, Mirei again decides against undressing with the rest of the girls. She ends up in a dressing room with the Shinomiya girls, who use her as a hostage to force the others to participate in a contest of who can stay the longest in the bath. In the end an embarrassed Mirei wins.
The next day the whole team, still feeling the effects from the endurance contest, performs badly at the tournament. Maaya in particular is shocked and feeling down. Mirei runs into her that night and comforts her. The next couple of days several members manage to qualify for the Inter-High tournament in Okinawa. Mirei, with Maaya, Sanae and Momoko, qualifies for the medley relay event.
In preparation for the Inter-High tournament the competing club members (and Kaname and Makio) hold a training camp on the Daito Islands, where Amuro’s family lives. There, Mirei tries to get closer to Kaname, and a couple of times they are together, though each time Amuro shows up. It is obvious Kaname and Amuro, even if they don’t realise it yet, have feelings for each other. One night they all go stargazing and during this Kaname is kissed by someone. Because of her behaviour (getting nervous and blushing) Kaname starts to suspect Mirei, but when asked she explains her behaviour being because she saw Kaname kissing someone.
During the preliminary medley relay at the Inter-High tournament when it is Mirei’s turn, as the third swimmer out of four, the team is in seventh place. She gains one position and Amuro gains a few more, finishing top three, qualifying them for the finals. There, they finish last due to Momoko participating despite being injured.
After the third years leaving the swimming club and Kaname becoming the new captain, a new vice-captain has to be chosen as well. After Kaori Himekawa volunteers, Makio makes Mirei volunteer as well, which leads to Amuro jumping in as well. In the end Mirei becomes the vice-captain, Amuro the substitute vice-captain and Kaori the vice-captain’s assistant.
When the girls decide to make chocolates for Valentines Day, Yamana Yoshizawa shows some aphrodisiac. Mirei puts some into her chocolates and later invites Kaname to her house. Kaname visits her, bringing with him her panties, which he hopes to put into her drawer. After being fed a few chocolates, he starts to behave as expected, until he hits his head. Sobered up, he notices he was hit by her collection of wooden dolls and a pair of his boxers, which Mirei had found one day.
Five years later she works for the Shizu Group’s main office and also secretly designs underwear.
She is a very good swimmer, who participates in 200 m and 400 m individual medley, freestyle relay, medley relay competitions.