Yuu (ゆう) is a local elementary school girl who frequently visits the Inokashira Park police box to annoy Saejima. She seems to share a deep connection with Saejima and understands his perverted nature, even stealing her mother's panties for him. She comes from a well off family and has a younger brother named Yuuhito.
Although never stated, it is likely that she is Chika's stepdaughter, since Yuuhito is Saejima's son and there was no mention of Chika already having a kid when they were together.