Keiichi is the youngest male competitor in the concours. He likes to do things at his own pace. He is very drowsy all the time and often seems confused or in a daze. Keiichi speaks slowly and politely, in a quiet voice, and can be found sleeping anywhere, anytime. Kahoko first met him lying on the ground, fast asleep. His life seems to revolve around music - he tells Kahoko that he sleeps, wakes up, plays the cello, then repeats the process, other than eating, going to concerts with his family and visiting the library to learn more about music. He is currently living at his aunt’s house. He enjoys reading as well as studying the history of music and structures of instruments. In time, he becomes interested in Kahoko because of her carefree music. He has an older sister, a younger brother and a younger sister. His older sister always sends him costumes to try on. Kahoko also commented that when he smiles, he looks like an angel. He also often calls Mr. Kanazawa the wrong name.

Source: Wikipedia and MyAnimeList

Aoi is one of the new characters that appears in the second game and was introduced later in the chapter 47 of the manga. He's a new student in the General Education department, who transfers to the academy and is sorted into Kahoko's class. Before he enetred Seiso, he attended a public boys school that was hard to get into and played tennis. He received several confessions to girls everyday but he refused them all. When he was at the park, he heard Kahoko playing and immediately fell in love with her. He came to that park everyday to come see her but he never saw her again until he saw her beside him while crossing the street. He followed her and he saw her meeting with Ousaki and Len to perform at a store's opening party. He disguised as a mascot so he can go inside and hear Kahoko play.] After he entered Seiso he saw Kahoko and kissed her hand. When girls confessed to him he would say, "Sorry, I appreciate your feelings but I am completely dedicated to Hino-san." He would also compliment Kahoko every morning. He previously played the violin but dropped it and now plays the viola because of an incident in the past. He first met Kahoko at the local park when he overhears her playing the violin and he was so captivated by it that he openly declares himself her number one fan. He admits that he doesn’t have confidence in his own performances since he lacked training, but he has very sharp hearing and can easily spot flaws in others’ performances. He is also very fast in tuning his instruments.

Source: Wikipedia and MyAnimeList[