Sumika

Sumika is a middle school student who met Yuuko in Chapter 112 of the manga, and is a classmate of Mihoshi Tachibana. Although she is seen only in the manga, Sumika makes a cameo appearance in the second ending along with Tanabo. Her parents are known to have a considerable amount of wealth which implies she is the younger sister of Kenzaburou Daiku. If so, this would mean that her last name is also most likely Daiku. In Chapter 136, she mentions that she has an older brother who's into some sport where people can die (ie, Igo Soccer).