Komachi Hikigaya

Komachi is Hachiman's younger sister who, different from her brother, is cheerful and lively, and is a member of the student council in her school. She understands that her brother has a corrupted personality but still cares for and relies on him regardless. While attached to her brother, she shows cleverness in trying to pair up Hachiman with Yukino or Yui.

Source: Yahari Wiki