Nanako Hazuki is the sister of Nagisa Hazuki.
Though Nanako has not been shown in the anime, Hiroko Utsumi has stated that she likely has shoulder-length hair, the same color as Nagisa's. She would be most often seen wearing a mint green headband and bracelet. Her eyes are light blue, unlike Nagisa's. Hiroko has also made it clear that this was just a quick thought of what she might look like.
Nanako acts much like Makoto Tachibana, and has a very calm demeanor. She is often annoyed with Nagisa when he begins to become loud or rambunctious, and will scold him. However, she can be the same way if she is excited enough. This is nothing but a thought, much like her appearance.