Maiko is Shuichi's sister. She never appears in the anime and she is only briefly seen in the OAVs. You can see more of her in the manga, where she was the first keyboardist of Bad Luck. Of course, she was later replaced by Suguru.^^ In the OAV, she's seen because Shuichi goes back to his house and shuts himself up in his room. Maiko comes and gives him food. She's very considerate and concerned about Shuichi when he refuses to come out of his room. She can be as genki as Shuichi, but she's more mature than him, being the older one^^ and would often complain about Shuichi's habits or actions.

Yuki is Shuichi's love interest, and Shuichi's somehow used his genki personality to get through to Yuki and cause Yuki to display affection for him. Shuichi loves Yuki with all his heart, as much as he loves singing. Yuki on the other hand, uses verbal irony quite a bit. Although Yuki's always insulting Shuichi and such, he really means the opposite and slowly falls for his cuteness. XD The two are pretty much complete opposites, but as they say, opposites attract. :D