Complex sentence: Kanako is a girl who at the beginning of "Hōrō Musuko" is a fifth grader in Shūichi's class, and is nicknamed Sasa-chan, but is also called by her younger brother and Chizuru.
Split into simpler sentences: Kanako is a girl who is a fifth grader in Shūichi's class at the beginning of "Hōrō Musuko". Her nickname is Sasa-chan, but she is also called by her younger brother and Chizuru.