Hawatari Yuiko (羽渡唯子)
On the surface, Yuiko appears to be a total ditz, but she has a good heart and a knack for picking up on concealed emotions. (That doesn't stop her from being oblivious to what's really going on between Soubi and Ritsuka, though.) Like Ritsuka, she doesn't fit in very well at school and has difficulty understanding the complicated world of adults.
In an interview with Puff, a Japanese magazine, Kouga Yun said that Yuiko was created to be the perfect girl for Ritsuka. Many fans see her as "the enemy" or Soubi's rival, but I don't think it's that simple. She's an important piece of Ritsuka's life, because she is the only normal friend he has. While Ritsuka may not pick up on it, she's openly infatuated with him, and tries her hardest to make him happy, even if it often backfires (see: the jam, the flowers, et cetera).