|Height||165 cm (5' 5")|
|Anime debut||The Scenery With a Carving|
|Japanese voice actor||Miho Saiki|
|English voice actor||Joanne Bossano|
Mitsui (三井) is a student of Hikarizaka Private High School who first appears in The Scenery With a Carving where she talked with Fuko Ibuki for a bit during the school's entrance ceremony before her accident. Though she remembers Fuko, she refuses to accept Fuko's starfish sculpture gift and request to go to her sister's wedding, since she is a senior and is busy with entrance exams. This hurts Fuko's feelings. In the visual novel, Mitsui is shown to be more antagonistic towards Fuko (rather than the mere indifference that can be taken from her anime interpretation); for example, she berates Fuko's continued attempts to distribute her starfish sculptures to the senior class that Mitsui represents, calling Fuko a nuisance to the other students' studies.
In The Older and Younger Sister's Founder's Festival, she finally accepts Fuko's gift after seeing that she continued to give them out to people. She then promises to go to Fuko's sister's wedding if it was after her exams. This cheers Fuko up.
However, in The Wind That Vanishes Into Dusk, Mitsui, like most people, starts to forget about Fuko. She thinks Tomoya Okazaki and Nagisa Furukawa were playing a trick on her when they try to make her remember Fuko. In Until the End of the Dream, she finally remembers Fuko and her sister's wedding and came to the wedding to celebrate it.
- The name Mitsui means "three" (三) (mitsu) and "well, mine shaft, pit" (井) (i).