"So tell me, what are you reading these days?"

- Sanae to Kōichi in "Blueprint".

Sanae Mizuno (水野 沙苗, Mizuno Sanae) was the nurse in charge of Kōichi's care during his stay at Yomiyama Hospital. She was also the older sister of Takeru Mizuno, which connected her to 9th Grade Class 3-3 in 1998.

.]] Sanae was a thin, fair-skinned young woman of average height. She had short light brown hair and large brown eyes. Most of the time, she was seen wearing her nurse's uniform, which consisted of a white cap, a white dress, a small dark blue sweater that she usually wore unbuttoned, and white sneakers. In "Put Flesh", she was seen wearing her casual outfit, which was a white striped dress and a pink t-shirt underneath.

In the manga, she has short black hair and large, black eyes, and wears the same outfit that her counterpart wears.

Sanae is a kind and cheerful nurse in charge of taking care of Kōichi when he is in the hospital. She suggested various mystery and horror novels to Kōichi to read while he was in the hospital, and also when they used to meet casually. Sanae is also shown as a sociable, friendly, and peaceful person. She is good friends with Kōichi. She tends to be helpful to others, especially Kōichi, assisting him in searching for information when he needs it.

In the manga, while her character is quite similar, Sanae appears to be clumsy as she walks into a wall and puts too much sugar into her tea. She also bites her tongue.

Sanae looked after Kōichi Sakakibara while he was sick in the hospital. She also helped him find information about the Misaki who died in the hospital, and even offered to give him her phone number. They talked about the class' curse and how everyone thinks that Mei Misaki doesn't exist. Sanae tells Kōichi to be careful because they don't know much about it yet. In "Put Flesh", she called him to ask if Mei really existed, since her brother claimed she didn't in a horrified manner while returning from the rooftop through the elevator. While she was talking to Kōichi about Mei, the elevator's motor started to spark and malfunction, and suddenly the cable snapped, causing the elevator to fall. Sanae is slammed into the elevator floor head-first, and she is crushed when the elevator collapses. This incident leads to her being one of the "Deaths of June". After her death, Kōichi is called in by detectives at his school to ask him a few questions about her death. Kōichi even questions if he is a suspect, but the detectives assure him it was an accident, as he was the last person to talk to Sanae, as her cell phone was found in the elevator and was looked at. In "Body Paint", she returns as a ghost in Kōichi's dream and appears when Kōichi opens one of the classroom doors. She lies on a bed with blood covering her body, blaming Kōichi for her death.

Sanae had several things in common with Kōichi and befriended him during his stay at the hospital. She seems to view him as a younger brother and often teases him. Kōichi was devastated by her death, especially in the manga. She was one of the ghosts who taunted him during his nightmare, suggesting he blamed himself for her death.

In the manga, Sanae flirts briefly with Kōichi, mostly to make him uncomfortable. This is only present during a brief inserted advertisement within the manga and is not actually canon.

In the film, Kōichi and Sanae aren't especially close. Kōichi takes Takeru's advice and stops involving her in his quest to learn about Mei, but this comes too late, and she dies anyway. Kōichi went to her memorial but was afraid to pay his respects as Takeru was present and glaring at him from beside the shrine.

Although Sanae says that she and her brother were not on good terms and did not talk often, they both still loved each other. Takeru repeatedly lied to his sister to protect her and warned Kōichi not to involve his sister with the curse. He blamed Kōichi for her death. In the manga, Izumi tried to use this to persuade Takeru to side with her when she confronted Mei at the inn, though Takeru wasn't willing to do so. However, in the film, Takeru was much more upset following Sanae's death, and in a rage, attempted to kill Kōichi at the inn.