She is the class president of Natsume's first-year class, Class 2.  Sasada suspects that Natsume can see yokai, although he repeatedly denies this to her. She moves to the city after her initial appearance in the manga due to her step-father's job, but is a recurring character in the anime as part of Natsume's group of friends.
In the anime, it is suggested that Sasada has developed a crush on Natsume following the events at the old school house, and much to his chagrin, frequently tries to accompany him when he is on business involving yokai, even though she hasn't been particularly successful.
Sasada wears glasses and has dark brown eyes and straight, shoulder-length brown hair. She clips her bangs to the side with a green hair-clip, and is most commonly seen wearing the school's uniform. In flashbacks, she is shown to have worn her hair in pig-tails.
As class president, Sasada acts authoritatively towards her classmates, and can be a bit bossy at times. Natsume notes that despite her complicated family situation, Sasada is a hard and efficient worker. As shown by how she looked for Shigure in the old school house every day to thank him for returning the amulet to her, Sasada greatly appreciates kindness, and is a very determined person who won't give up once she sets her mind to it. Her persistence is also demonstrated when she frequently follows Natsume around and asks him about what he's doing, possibly because she has feelings for him.
Sasada's mother had died when she was young. Her father eventually remarried, only to pass away soon after as well, leaving Sasada to live with her step-family.
During the Trial of Courage event at the old school house the previous year, Sasada was terrified, and in her hurry to leave, dropped an amulet given to her by her mother before she died. She rushes back to the school house and searches frantically in the dark, and meets a mysterious man wearing a bucket over his head, who is later revealed to be Shigure. Sasada explains her situation to the stranger, who tells her to hurry up and leave once she finds her amulet. She continues to search to no avail, when the man reappears and hands her the amulet on a stick, explaining that "The unclean cannot touch the blessed pure."  After being unable to find anyone with the man's voice at school, Sasada concludes that he must have been a ghost. She begins to visit the old school house every day, calling out to him in hopes of being able to thank him properly for his kindness.
Sasada is one of the organizers of the year's Trial of Courage, which will be the last since the old school house is going to be torn down. She pressures Natsume to stop being so anti-social and go to the event, but in reality wants him to go since she had been watching him and has suspicions that he can see supernatural spirits. When the Trial of Courage goes awry as students go missing, having been taken away by Shigure, Sasada confronts Natsume about her suspicions regarding his supernatural abilities. Natsume tells her that he has no idea what she's talking about, but when she continues to persist, Nyanko-sensei knocks her out. When Sasada wakes up, she apologizes for her rude behavior towards Natsume and tells him about the mysterious bucket man she had met before, revealing that this is her last chance to ever see him, as she is moving to the city next week due to her step-father's job. Natsume realizes that the bucket man must have been the yokai who had attacked him earlier, and Sasada overhears Nyanko-sensei, having now transformed into his human form as a Reiko look-alike, telling Natsume that the yokai is in the Book of Friends, and that his name is Shigure. Sasada tells Natsume that if he can see yokai, she wants him to let Shigure know how much his act of kindness meant to her, and how that kindness has given her courage every day since.
More yokai start coming after them, so Natsume tells Nyanko-sensei to keep Sasada safe, and rushes to the rooftop to meet Shigure. He sees that the students are only sleeping, and Shigure means no real harm. Shigure does not want his name back, saying that it has been corrupted by evil, but as Natsume returns his name, Shigure is reminded of his act of kindness towards Sasada, and his memories flood into Natsume. When Sasada came to the old school house looking for him day after day, Shigure had grown fond of her but never showed himself, fearing that once she saw him, she wouldn't come back again. Natsume relays Sasada's message to Shigure, telling him that he is not unclean at all, and Shigure overcomes his hatred of humans and is able to rest in peace. As he starts to fade away, Sasada arrives on the rooftop looking for Natsume and Shigure touches her at last, bidding her farewell. Although she cannot see him, Sasada can sense a connection from her heart, and the incident remains a pleasant memory. In the manga, Sasada moves away the next week.
Natsume drops an old ticket he found among Reiko's belongings, and Sasada hands it back to him. She asks him about the story behind the old ticket, and can tell that he's lying when he says that he just found it on the way to school. Natsume heads to the bus-stop after school, and finds out that Sasada followed him. She asks him where he's planning on going with the ticket, and when Natsume asks her why she cares so much, Sasada blushes, and looking for an excuse, says that ever since the Trial of Courage, she has always wanted to see "one", as in a yokai. Tanuma arrives and manages to distract Sasada long enough for Natsume to escape on the bus.
- Main article: Shigure
- Main article: Takashi Natsume
- Main article: Tooru Taki
- Main article: Kaname Tanuma
- Main article: Atsushi Kitamoto
- Main article: Satoru Nishimura
- (To Natsume) "I have to look for Lord Shigure. This might be my last chance I have to see him. He may hate people, but he still helped me when I need it the most... I have to tell him how much my amulet meant to me. How happy I was that he helped me to find it. How reassured I was simply by his presence. He has to know - he's not unclean at all. I have to tell him his kindness gave me courage, not just then but every day since." 
- (Natsume, in narration) "Sasada, I saw Lord Shigure reach out to you for a moment. I saw him try to make a connection. Did you feel it? I'm sure... You must have felt it." 
- Sasada's first name, Jun 純, means pure and genuine.