Kaori is a violinist that was also inspired by Kosei when she was younger. Originally, she was a pianist. But, after seeing Kosei's performance, she dropped the piano to play the violin because she has been dreaming of playing alongside Kosei on stage
Kosei is the main character of the series since the series' story revolves around him. After his mother's death, his ability to hear the piano goes along with her.
Hiroko is the piano teacher of Kosei and she was initially the one who introduced Kosei to the joy of playing the instrument. The first time she discovers Kosei's natural talent, she encourages him and his mother to be tutored by her.
Even though so many viewers might hate the mother of Kosei Arima for being the reason Kosei lost his ability to hear his own piano because she often portrayed pushing Kosei harder and harder to the point where she abused Kosei sometimes, she actually has good intentions on doing so
Just like Kosei, Takeshi has been playing the piano from a young age and practices every day. He was inspired easly on the methodical and masterful performances that Kosei puts on and Takeshi dreams of surpassing Kosei.
Emi has similar roots to Takeshi when it comes to Kosei. She saw a performance of Kosei while she's younger and gets inspired by the performance. However, Takeshi sees Kosei as a rival, meanwhile Emi sees him as an ideal. S
Tsubaki is one of the support characters in the series and also one of the best Your Lie in April characters. She has been best friends with Kosei since they were young. She has seen his rise and fall as a musician and stayed with him every step of the way
Nagi is the younger sister of Takeshi Aiza. She initially despises Kosei because of her natural loyalty to her brother. As the story goes on, Kosei becomes her tutor and Nagi seemed to be unhappy about it.
Many of you might not remember Saitou at all. Saitou is the guy that Tsubaki dates on the majority of the series. She confesses to him as they started dating. However, Tsubaki is actually in love with Kosei.
Watari is the playboy character of the show. He is the best friend of Tsubaki and Kosei as he tags along the two on the series. Although he is being used by Kaori to fulfill her greater ambition, he presents a very important message to us all.