Makiya Sumika♀

Yumimi's classmate who is also the younger sister of Kasumi, whom she lives together with due to their parents always working. Like her sister, she has known Shoutarou for a long time already and calls him "onii-chan". Loves her sister very much, but is also quite rude and doesn't understand why Yumimi continues participating in the swimming club despite never yielding better results. Combined with her competitive and loud personality, Sumika is also a very energetic girl.