- "So rather than Katsura-san I'll be the one who will bring you happiness"
- — Sekai to Makoto after admitting that she stabbed Kotonoha
Sekai Saionji is the main character of School Days manga.
Sekai is a average built girl with a shoulder length brown hair with a small amount from each side tied together at the center of the back of her head. Her eyes are blue. She is normally seen wearing a school uniform and later on during the culture festival she wears a maid outfit. Her casual attire includes a pink jacket with a white shirt, blue denim mini skirt and a pair of boots. During summer at the beach her swimsuit is a light pink bikini and a white skirt. During the festival she wears a red yukata.
At some point at the beginning of the semester of school Sekai spotted Makoto and did the charm. But to no avail after waiting so long.
Sekai was first seen when she surprisingly asked Makoto if the picture on his cellphone was his girlfriend, severing startling Makoto. She swipes the phone from him and thinks that Makoto is doing the charm and catches him off guard before he demands she returns it. Sekai teasingly refuses until Nanami calls her and she grabs Makoto's head in front of them, humiliating the latter. Sekai returns Makoto's phone as Nanami realizes Makoto knows the charm. After he accidentally gets close to Sekai they resume teasing him. Setsuna joins Nanami and they consider their moment to be a couple's skin-ship, which Makoto tries to deny, along with Sekai until the Teacher arrives. During class Sekai tells him that she knows the person in his phone, then offers to work with him, causing Makoto to cast suspicion.
At the rooftop Sekai was confronted by Makoto about her offer to work together. Sekai tells him that he likes Kotonoha and due to messing up his charm, she wanted to make up for it. Despite Makoto's initial refusal, she convinces him by telling him that just taking a photo of her and hoping for the best will not fulfill his love for Kotonoha. She also points out the possibilities that Kotonoha already had a boyfriend and tells him to leave it all to her before she resumes teasing him until he agrees. Sekai and Makoto started to spy on Kotonoha on that day from the hallway, to the canteen, and to the bathroom (with Sekai making Makoto stay out of it). On the rooftop, Sekai tells Makoto that she shares the same P.E class with Kotonoha and pretended to be ill in order to speak to her, as she wasn't feeling well and sat to the side during class. She reveals to Makoto that despite how cute Kotonoha is, she hasn't made any friends yet, then reveals she was able to befriend Kotonoha. At the locker room, Sekai is surprised to see how huge Kotonoha's breast are - incidentally scaring her after she pokes them. She reveals this to Makoto before pointing out that she also invited Kotonoha to have lunch with them.
Sekai arrives first with Makoto and he panics and protests that he is unable to go through with this. She insists that this is an opportunity for him, just as Kotonoha arrives. Sekai introduces them to each other and they sit down for lunch.
After class Sekai asks Makoto if he had confessed to Kotonoha yet, but he refuses to answer, thinking that she may like someone else. Sekai reveals to Makoto that she actually doesn't like anyone and has a strict Father. She does point out that Kotonoha did have her eye on someone, but she tells Makoto not to give up. Makoto is unsure though, as he worries that if the confession goes badly, things will just return to how they were before. This infuriates Sekai and she slaps Makoto, repeating that his love won't come true if he just wishes for it, then she runs off while calling him a spineless coward. Makoto catches up to her and thanks her for her help, then goes on to make a resolve.
Sekai and Makoto walk to the station together and he reveals that his confession to Kotonoha was a success. In return he'd like to do something for Sekai, but the train arrives. She takes advantage of this and kisses him, then wishes him luck after running to her train and taking off. She is seen to be distraught for what she did afterwards.
The next day, Sekai laughs at Makoto for being yelled at by the teacher for not paying attention. After class she insults him by calling him a "Class Dunce", and guesses that he was thinking about his date with Kotonoha. Makoto asks her about the day before when she kissed him however, but Sekai avoids answering by changing the subject. Makoto keeps asking about the kiss, but she stops him by calling out his behavior and reminding him that she is only trying to help him with Kotonoha, as she knows they will need to start distancing themselves. With sarcasm, Makoto thanks Sekai and addresses her with "sama", effectively annoying her.
At the hallway Sekai and Makoto discussed his progress with Kotonoha and admits to being nervous with her, and hasn't even called her by first name yet. Sekai argues with him about this, saying that he will be unable to go any further with this relationship if he still acts awkwardly. As she continues to yell at him, Makoto notices that Sekai has mentioned kissing and further romance, then eventually to the fact he is ignoring her. He begins to muse over something until Kotonoha calls for them, bringing them with her to have lunch on the roof like usual. They chat about a message but things get awkward for Sekai and Makoto again until Setsuna invites Sekai to have lunch with her and the others in the cafeteria.
At some point, Sekai advises Makoto to make his move during a date with Kotonoha, and he tries to hold hands with her. This seems to bother her though, as she dislikes holding hands with boys.
Sekai asks Kotonoha about this in the Girls Locker room the following day. She recalls what Makoto told her and brings it up, with Kotonoha admitting that holding hands with boys tends to scare her. She seems depressed until Sekai calms her down, admitting that she had given the advice to Makoto in hopes of getting them closer together.
At the school rooftop Kotonoha arrives on time and sees Sekai comically strangle Makoto, who wanted to have a bite of Sekai's lunch despite the latters protest. She notices how close they are until Sekai spots her and invites her to join them.