Mizuka Irei (伊禮 瑞榎) is the owner of the Irei family convenience store and is a childhood friend of Misaki Senomiya. She is very knowledgable about Tanegashima, its residents, and rumors. At her store, Mizuka sells a special, passionfruit nikuman which is considered to be extremely repulsive and anyone who wishes to receive Mizuka's advice or dip into her vast knowledge must first purchase and eat one. This ritual is known as the "Lake Treaty." Mizuka is also paraplegic and wears a Power Suit leg supporter to help her walk.
Mizuka is rather tall, a bit taller than Kaito, and has blue eyes. Her long green hair is tied back in a ponytail and her bangs are separated by a hair clip. She is well-endowed and typically dresses in a simple tank top and jeans. Mizuka also fashions a Power Suit device on her legs, which enable her to walk.
Mizuka was a classmate of Misaki's and the two were close friends. She was the only person to learn of Misaki's murder of Ko Kimijima and hid the secret from Akiho and Kaito. After graduating, Mizuka took it upon herself to watch over Kaito and Akiho.
At some point later, Mizuka was involved in a motorcycle accident which left her disabled from the waist down. She struggled to deal with her disability but years later, she would receive a present from her old friend, Misaki. The present was a Power Suit gear for her legs, which would give Mizuka the ability to walk once again.
- Misaki Senomiya: A close friend and former classmate in high school. Mizuka was the only person to learn of Misaki's murder of Ko Kimijima and hid the secret from Akiho and Kaito. Mizuka originally recieved her Power Suit from Misaki and is very grateful to her old friend. Over the course of the series, Mizuka keeps in contact with Misaki, encouraging her to regain contact with her younger sister.
- Kaito Yashio: After Misaki graduated and left Tanegashima, Mizuka become a sort of sisterly figure to Kaito, diligently watching over him and Akiho. Kaito frequently comes to Mizuka for assistance and advice, but always at the cost of eating one of her nikuman. It seems Mizuka's aloof disposition rubbed off on Kaito somewhat.
- Mitsuhiko Nagafukada: Mitsuhiko was a former classmate of Mizuka's and the two are shown to retain a close friendship, though Mizuka considers him tiring to deal with.
- According to Mitsuhiko, Mizuka is fond of beer.
- Her given name, Mizuka, contains "Mizu", which can mean "Water." Her family name, Irei, can mean "Memorial."