- When Yumi first runs into her in the student council room, Sachiko is portrayed as someone who is a far cry from the perfect lady that Yumi pictures her to be (this might not be apparent in the anime and manga, as the novel has a more detailed account of the incident). In the novel, Sachiko somewhat knew Yumi is her fan and was overconfident that Yumi would never refuse her.
- Though prim, elegant, and somewhat aloof, Sachiko has both a rough (the novel and manga affectionately terms this as her "hysterical side") and a very sweet side. She is very protective over Yumi and cares deeply about her. However, Sachiko often has the same problem as Yumi does—she does not share most of her problems with her sœur, causing many misunderstandings. Sachiko is slated to marry her cousin and fiancé Suguru after she graduates. This has caused her some distress since, because although she initially cared deeply for him, he did not reciprocate. They now have a complicated relationship.
- Sachiko also cares a lot for her peer, Rei, especially with regards to the latter's conflicts with Yoshino - she once went all the way to beg with Rosa Chinensis and Gigantea (even going into her hysterical mode) to help out; she gives Rei a hug when the latter is unsure of how she could balance between her "own future and own life" and "life with/without Yoshino" during the latter parts of the series.
- Sometimes, Yumi must teach Sachiko how to live informally, for example, helping her select her first pair of jeans and eating in a fastfood restaurant. Despite her sometimes stern appearance, Sachiko has her own share of insecurities regarding her relationship with Yumi, not to mention emotional insecurities rising from her home life; both her father and grandfather openly have mistresses, unabashedly dividing their time between home and the mistresses, which apparently led to her general disdain for men, as illustrated in the OVA series during a meeting with the student council members of Hanadera, an all-boys' high school.
- After Sachiko enters her third year, she is now the new Rosa Chinensis. After the arrival of Tōko, Sachiko spends more time with her than with Yumi, which makes Yumi jealous. Sachiko goes absent from school which makes Yumi worried. Later, Yōko is sent to "bring" Yumi to Sachiko's grandmother house. Yumi finds out that Sachiko's grandmother recently died, which causes Sachiko to go into depression. Sachiko says that she apologizes for the past few weeks, and asks for Yumi's forgiveness; Yumi says that she loves her very much.
- At the beginning of the summer holidays, Sachiko takes Yumi to her summer home. Whilst there, Sachiko's acquaintances make trouble for them. Yumi intends to leave, but Sachiko refuses. The acquaintances have a party, and invite them. At the party, they ask Yumi to play a musical instrument for one of the acquaintances great grandmother. Yumi says that she cannot play anything and prefers to sing the song "Maria-sama no Kokoro"; Sachiko accompanies her with a piano performance. The great grandmother is happy because the song brings back memories, and she asks Yumi to visit next year, foiling the acquaintances' plans for her humiliation in the process.
- In the fourth season, Sachiko is already asking Yumi to look for a petite sœur, although she is still deeply concerned about Yumi, especially when one potential candidate for the future Rosa Chinensis en bouton rejected Yumi, although at first.
She is a "Japanese Princess" as she comes from a family of great wealth and status. She is the eldest daughter of the Ogasawara family which makes her responsible for marrying Kashiwagi Suguru. She said that she is supposed to marry him because his family has enough influence to run their company.
She is titled as the Lilian Joshigakuen's (Lilian All-Girls School) idol and star.
Former Rosa Chinensis, her grande soeur, Youko, chose her as petite soeur because she wanted Sachiko to be free. Free from her family restrictions, from the expectations of others and be herself without holding back. Youko also made Sachiko quit from all of her activities such as ballet classes, lessons and such.
Sachiko is often described as being cold and aloof but later on becomes a bit carefree after being soeurs with Yumi. She picks up some lines and habits from her grande soeur, Youko, and delivers it to Yumi at particular times in the same manner and words.
She is also known to be a stubborn woman as stated by Youko. Sachiko does not give in easily to her grande soeur orders or decisions without putting up a fight. However, she seems to be often persuaded by Yumi regarding her decisions. For example, in the first season, she asks for Yumi's comment about the dress in the play, Cinderella. She completely disagrees with the others of the Yamayurikai but hearing Yumi's comment makes her decision the other way around. So basically, she barely argues with Yumi than with Youko regarding to school activities.
Her stubbornness seems to have been inherited from her father, Tooru, as well as his sense of humor. While she had inherited her strictness and lady-like manners from Sayako, her mother.
Youko once stated that Sachiko does everything that is expected of her. Also according to Sei, Sachiko is an honest person, staying true to herself. She is a frank and straight to the point, she naturally speaks what's on her mind even though it may sound rude but Sei states that it's actually a good point of Sachiko's personality. Although, there are times that there are some things that she keeps to herself until the right time comes. Sometimes, she is hard to read because of her blank expressions that even Yumi could not read what's on her mind all the time but she could read Sachiko's emotions, though unsure, at the moment.
Losing is what Sachiko hates the most. She also hates the the place where sakura and gingko trees grew where Shimako and Yumi often had lunch. She hates crowded places and though she did not state that she hates summer, she says that the heat does not cooperate with her that causes her to go to the villa instead. She also hates people who are desperately trying to give her chocolates but Yumi is an exception. She also hates people that are fake, values their names and status in life as well as people who are desperate "wannabe's".
She is quite protective of Yumi even from the other people, not caring about their social status as seen in the series during summer vacation at the Ogasawara vacation house.
She easily gets jealous of Sei whenever she sees Yumi to be comfortable and enjoying with her. Though she may sound angry, she either scolds Sei or Yumi to release her jealousy. Sometimes, she evades the situation and makes an excuse to leave but in reality she's just really jealous. Also when she is embarrassed, she acts a little angry as noted by Yumi.
She has misandry. (Misandry means hatred towards men in general.) Though she doesn't seem to hate Yumi's brother, Yuuki. She had also gotten rid of her hatred to Kashiwagi but the other men such as the members of the Hanadera Student Council were another case. However, after their meeting, her hatred seems to have eased.
She is also acrophobic or has a fear of heights. Rei had told Yumi that when they were in a field trip where they took a plane, Sachiko kept on drinking pills, eat, sleep and repeats the cycle again. However, her fear must have been overcome since the event of the Hanadera school. When she had noticed a running mascot in the crowd, Rei, Shimako, and Yoshino were greatly surprised that Sachiko swiftly climbs down a two-story ladder without hesitation despite her acrophobia just to meet Yumi, wearing the panda costume. Yumi was surprised as well that her Onee-sama knew it was her without making any mistake.
She also has a fear on moving cars. Yumi had noticed when they were heading to the villa, Sachiko slept throughout the ride.
She does not know of the "way of the common people" like shopping or eating at a fast food restaurant because of her status until her "date" with Yumi.
Kashiwagi Suguru He is Sachiko's cousin as well as her ex-fiancee -an arranged marriage by the Ogasawara family. At first, she shows her deep hatred of Kashiwagi but in reality she was revealed to have been in love with him. However, Kashiwagi did not love her in return and she was rejected which caused her to be heartbroken and bitter about the engagement. Kashiwagi had admitted to her that he couldn't have any romantic feelings for her because he was only interested in men and that he told her that she was allowed to have an affair and any child that comes out of it would be considered their heir. She was originally supposed marry him at the age of eighteen after she finished high school in Lilian. However, Sachiko eventually ends up breaking their engagement when she realizes how unfair the arrangement was to both of them and only after gathering enough courage and resolve to confront her parents about it. Their strained relationship becomes better after all of the tension because of the engagement diminished after marriage was called off. They are shown have grown to have platonic brother-sister relationship after the air is cleared between them.
She is Sachiko's petite soeur. They first met in front of Maria as well as they became soeurs. Yumi was the second person to refuse in becoming Sachiko's soeur but later on changed her mind.
Before Yumi accepted her rosary, at the back where a sakura tree grew between the gingko trees, Yumi and Shimako were having their lunch. Unexpectedly, Shimako was surprised to see Sachiko at a place where she hates just to say to Yumi that she is determined to make Yumi her soeur.
Yumi was the only person who could make her laugh without holding back or even change her mind without an argument.
Sachiko states that Yumi was the only person that could handle her well. Not even her grande soeur, Youko, had handled her better than Yumi could.
She is the reason Sachiko had changed for the better. From Sachiko's cold and reserved personality, she became open and warm to Yumi and the Yamayurikai. She smiles more often, became funnier and a bit of a joker that somehow surprised the others.
She is very protective of Yumi that she even loses her anger to anyone that tries to badmouth her petite soeur. She states in one of the OVA that she doesn't care if she's being embarrassed by others, she also says that she could never be ashamed of Yumi. She was just worried that Yumi might get hurt because of her.
While Sachiko was throwing off her tantrums, Yumi thinks that she is surprisingly calm that she even wonders why. Although she does not realize it, she is actually very happy that her Onee-sama is mad.
It was also seen earlier in the episode, while Yumi and Sachiko were arguing about their promised date in the amusement park, Shimako and her soeur had asked Rei and Yoshino if they were supposed to stop them. Rei and Yoshino says that they were merely having a "Harmless-Sisters-Quarrel" and that they are just joking around. By the time Sachiko had emptied her cup, Yumi stands and offers another cup as if they were back to normal again. Shimako and Noriko were surprised at the sudden change of their behavior, not understanding anything.
Sato Sei Edit
She is most jealous of Sei because of her closeness to Yumi as seen in the first and second season. Whenever she comes inside the Rose Mansion, she sees Sei hugging Yumi or even kissing her on the cheek making her quietly mad at them and even scolds both of them. Even though Sachiko is known to be a person with self-control, Sei can easily provoke her by teasing Yumi which was a kind of entertainment to Sei. Even so, she does not hate nor dislike Sei.
Hasekura Rei Edit
Rei seems to be the one of the closest person to her including Youko and Yumi. They are most probably closest of friends because they had confided to each other about personal problems such as the Rosa Foetida family incident when Yoshino had returned her rosary.
Mizuno Youko Edit
When she became her grande soeur, Youko had Sachiko quit all of her duties and activities that were expected of her by the Ogasawara family. Youko wanted Sachiko to become free from her family traditions even if only for a while and to live as her self.
At first, Youko did not deem Yumi worthy to be Sachiko's soeur but it seems that her inhibitions were wrong when Sachiko had greatly changed for the better because of her.
Youko had stated that Sachiko is stubborn and is often shown that she always opposes to her decisions. Although, Youko always wins the argument.
- She has been wondering if the Rosa Gigantea family has some kind of sickness or tradition that the members are awfully taking too long in choosing their soeurs.
- In the Marimite Live Action Movie, her role was depicted by Haru, a fashion model and an actress.
- After she graduates high school, her plan is study at Lilian University which is located right beside the Lilian All-Girls School. One of her reason is that she'll be able to stay close to Yumi despite her strict schedule.