- The exact opposite of most of the characters in the series: loud, rough-around-the-edges, she loves doing what she can to push other peoples' buttons. She especially finds joy in teasing Yumi (and also enjoys irritating Sachiko in the process) as she likes to see their reactions; Yumi affectionately calls that her "old man" behavior. However, Sei is one of the wisest and most caring characters in the series. Though seeming to have a playful and laid-back attitude, Sei has experienced many tragedies in her life, especially in her second year. Sei's personality was very different in her first two years. She was uninterested in everything, and never joked around like she did in her third year. Sei believes that Shimako should learn to solve her problems on her own, then only will she become strong. After leaving Lilian high school, Sei still appears as a secondary character.
- While the relationship between Sei and Shimako seems cold, the truth is Shimako is extremely devoted to Sei. Their relationship, as Yumi noted, is something that cannot be easily understood. After graduating from the high school division of Lillian, Sei studies English and American Literature at Lillian University. Sei shortens her hair when she goes to study at the university, as indicated in her appearances in the fourth season. She is initially the only one among the Yamayurikai who can drive. She once took Yumi for a ride in her car.
Sei is a half Japanese.
Her grande soeur, which was not stated in the series but was only shown said that she chose Sei to become her petite soeur because of her beautiful face.
She's always been said to have a devil-may-care attitude, but it could be seen as a facade to hide her true troubled self. She's also quite sentimental about some things like when she graduated from high school.
She hates gingko nuts and the greenhouse garden -probably because it reminds her of Shiori.
It was shown once in the series that she's also temperamental especially when Youko interferes with her and Shimako. But later in the series, she somehow gains self-control.
She also once stated that despite staying in Lilian, which is a Catholic school, she has a hatred to Maria for taking Shiori away from her.
Even though she has a laid back attitude, she deeply cares about everyone of the Yamayurikai especially Sachiko, Yumi and Shimako.
Sei underwent two transformations during the course of the show.
During her first two years, she had a longer hair than she did in her third year but she then cut it.
In her college year, she's shown to have a shorter hair like Rei's. The only difference is that Rei's hair have side bangs while Sei's have full bangs. Other than that, there was also a slight difference in their hair color.
Sei's grande soeur states that Sei has a beautiful face that she chose her.
During her second year she was involved in an affair with a first year student named Shiori, who later left to become a nun.
It was also said that during her first two years in Lilian, she wasn't good in making friends nor mingling with other students. When she entered her third year, she becomes carefree with the others especially with the Yamayurikai.
Mizuno Youko Edit
Youko and Sei are said to be good friends as shown in the 2nd season of Marimite. Sei tells Yumi that Youko had told her many things about the Ogasawara family traditions and such.
Youko was the one to whom Sei had opened up to since they met during the first two years. Sei at first hates Youko for barging in suddenly but then had dissipated and became good friends with her, probably the closest of friends.
It was implied that Youko has unrequited feelings for Sei however, even though Sei knows about it, she chooses not to reciprocate.
Ogasawara Sachiko Edit
Sei likes to irritate or even make Sachiko jealous especially when it comes to Yumi. During the 1st season, Sei often teases or make fun of Yumi whenever Sachiko is coming to the Rose Mansion, specifically, the conference room where each member of the Yamayurikai meets. Because of this, Sei was scolded by Sachiko not to pamper her soeur anymore but Shimako instead.
During the 2nd season, inside the Ogasawara Mansion, Sei had made Sachiko jealous by feeding Yumi a takoyaki that she bought earlier that day.
Despite this, Sei cares about her like everyone else in the Yamayurikai.
Fukuzawa Yumi Edit
Sei often makes fun of Yumi during the series by hugging, kissing on the cheek, etc. She also pointed that Yumi makes funny faces whenever she teases her which she also tells to Youko.
During the planning of the farewell party for the grande soeurs, Sei told Yumi of an untold tradition of the 1st years of the Yamayurikai, a performance. She said she wanted to laugh from the bottom of her heart before graduating that's why she tricked Yumi into it.
Just before the graduation ceremony, Sei was alone in her classroom until Yumi came in. She asks if she could do anything for her. Sei asks her for a kiss. Halfway, Yumi refuses. Though her request was supposed to be a joke along with her threatenings, Sei was utterly surprised when Yumi kissed her on the cheek.
Despite being a bully to her, Yumi mostly shares her personal problems to Sei, particularly when it is about Sachiko.
When Sachiko and Yumi had a misunderstanding during the Valentine's day event, while Yumi was sulking in the greenhouse, Sei unexpectedly comes despite her dislike of the greenhouse garden. This shows that Sei deeply cares for Yumi.
Todo Shimako Edit
Shimako and Sei are said to have a complicated relationship of being soeurs as stated by Yumi. Sei does not tell anyone about her reason why she chose Shimako as her petite soeur either. But even though their relationship looks complicated, both of them could understand each other the most.
It is hinted that Sei and Shimako are somehow opposites in other perspectives. For example, Shimako likes gingko nuts but Sei greatly dislikes them. Shimako is also known to be a responsible person while Sei is laid back and an I-don't-care-about-classes person.
In spite their differences, Sei and Shimako also have something in common like personal experiences and problems. They both sought for someone to understand them. In one of their meetings, Sei also stated that Shimako does not only resemble her but she was her herself.
She might not have known anything about Shimako but despite that, she was determined to make Shimako her soeur.
Kashiwagi Suguru Edit
Sei and Kashiwagi are revealed to be on bad terms. She hints that she dislikes Kashiwagi the most during the New Year inside the Ogasawara Mansion. Some says they are polar opposites and some, similar to each other when it comes to their personalities.
Sei also shown to be protective of the others from Kashiwagi like when she was protecting Sachiko or Yumi from him. It seems that her trust of him had dissipated when she saw her with Sachiko during the Cinderella practice in the 1st season.
- In the Marimite Live Action Movie, her role was taken by Takizawa Karen, a fashion model and actress who is a half Japanese and a half Russian.
- Sei is a very fast reader because she spent a lot of time alone reading prior to her third year at Lillian.
- Some people who watched Marimite had mistook Sei to be Rei ever since Sei went to college because they had the same look and almost the same hairstyle.
- During the entrance ceremony in her college, she misheard Katou Kei, a schoolmate of hers, as Satou Sei causing a great embarrassment to her.
- During the field trip to Europe by the second years. Sei went to Europe as well but did not show herself to Shimako and the others. It was still unknown why she went to Europe.