Mizuki is a girl full of friendly relationships inside of school and out. However, Mizuki might have a "certain" relationship that passes simple friendly passion.
Mizuki's older brother and the one she probably cares the most for. Mizuki seemed to cling to Nariyuki when they were younger. Mizuki gets infuriated whenever Nariyuki is with any other girls.
Mizuki first met Rizu when they came to visit in chapter 3 and was in awe of the talent of her cooking. This made Mizuki even more jealous of Rizu.