All right, they're not all really minor characters. I just don't have that much to write about them ^^;;;.
Mr and Mrs Arima

These are Arima's foster parents. They a relatively old wealthy couple. They feel that Arima is a very good son, who has given them everything that they could want. In return they have only praise for him. In Yukino's opinion they are very nice people.
Kawashima (Teacher)
Voice actor: Motomu Kiyokawa
After Yukino and Arima became couple their exam results fell considerably (well, Yukino's anyway, 1 all the way down to 13). As their teacher, Kawashima felt that he should talk to them. He did, in fact, ask them to break up. However, after talking to both them and their parents he supported them, saying that perhaps he was out of touch with what modern students think.
Ms Hiromi Ikeda
Voice Actor: Tagame Tamura
Ms Ikeda is the fiance of Bichichi Shibahime. She is a nurse, and has a son called Kazuma. She has been married before, but her former husband is deceased. She tries to be nice to Tsubasa, so that she will accept her. However, when Tsubasa visits the Ikeda house, and Ms Ikeda finds that it is messy she becomes angry at her son for letting Tsubasa see such a dirty house. However, instead of being upset by Ms Ikeda's pretence to be nice, Tsubasa accepts her, feeling that she is a lot like Yukino used to be.
Toshiharu Shibahime
Voice Actor: Hisoka Yamamoto
Tsubasa's father. He is married to Hiromi Ikeda. His previous wife is dead. Before marrying Ikeda, he lived alone with Tsubasa, and was submissive towards her.
Kazuma Ikeda
Voice Actor: Akira Ishida
Kazuma is Hiromi Ikeda's son. He is a member of a rock group, and has died his hair blond. Although he looks like a punk, he is a very nice person, and when he saw Tsubasa being attacked he stepped in to help her. When he was first introduced to Tsubasa by his mother, he was very happy that he was going to have a cute sister. Once he told Tsubasa that he has always wanted to have brothers.
Mr. Kohashi
Voice Actor: Yonehiko Kitagawa
Yukino's grandfather (on her mother's side). He does not get along with her father, and they quarrel all the time. He does not seem to appreciate that he has such a nice son in law. However he is very nice to his grandchildren, and dotes on them.