Bonnon is the villian in the SuperS Special Ami-chan no Hatsukoi (Ami's First Love).

During the special, Ami is feverishly studying for high school entrance exams, competing with a mysterious rival named Mercurius, who matches her perfect scores on every test. Ami works so hard that she begins severely lacking sleep. At her prep school one day, she accidentally sleeps through the entire class. When she awakens, it is dark out. Through the window, we see our first glimpse of Bonnon - a mysterious glowing red face which quickly disappears in a lilac mist. Ami leaves her class, quite disoriented. Meanwhile, the lilac mist materializes into Bonnon, who believes that Ami is in love because she is only thinking about a man - her rival. As a spirit, she plans to feed off of Ami's passions. She envelops the girl in her skirts, and Ami begins feeling sick on her way home. At home, Ami lies in the bed, ill, and troubled by thoughts that Mercurius is causing her harm. Because she is only thinking of Mercurius, Bonnon sees her opportunity to attack. She uses her "Love Letter Attack" against Ami. Suddenly, bubbles form over the girl's head, filled with all of the information she has learned and memorized. They begin to pop, causing her to forget everything. Meanwhile, Usagi, Mamoru, and ChibiUsa arrive at Ami's apartment. Inside, Ami is still having all of her knowledge sucked away by Bonnon. Usagi runs in to help her friend, but the floor has disappeared, leaving Usagi clinging to the door frame in desparation. ChibiUsa demands to know the villain's identity, but before she can introduce herself, the delirious Ami states that she is Mercurius, trying to defeat her. Bonnon tries to interhect with her true identity, but Ami won't allow it. She quickly transforms into Sailor Mercury and destroys the evil spirit with "Mercury Aqua Mirage."

(Source: soul-hunter.com)