Tomoko is Nanako's childhood friend

Tomoko is Nanako's childhood friend and arguably the only "normal-themed" character in the series. In the manga, Mariko manipulates her into ending her friendship with Nanako, which Tomoko later rekindles when she realizes the misunderstanding. In the anime, Mariko attempts to end the friendship and almost succeeds, but Tomoko ultimately remains by Nanako's side and protects her from Aya Misaki's bullying, and the three of them become best friends for the entire duration of the show, with Tomoko acting as the series' main comedic relief.