Ookura Lumine♀

Saika's childhood friend and the "older sister" type who is actually younger than him. However, in the sense, Lumine is actually Saika's grandmother; Yuusei’s grandfather (aka, Saika’s great-grandfather) is still alive and kicking at over 100 years of age, and somehow hooked up with a lovely 18-year-old chick. This relationship gave birth to Lumine. In reality, this is used more as a form of ridicule than respect, as she is called various names from “Great Aunt” to “Grandmother” to “Anego”. Despite this strange and messed up family tree, Lumine and Saika get along very well, and can be considered “osananajimi”.