Tsubaki (澤部 椿 Sawabe Tsubaki) is Kousei Arima's childhood friend who has always gone to his performances and cheered him on. Tsubaki would often invite Kousei to play outside but due to his practice time with his mother, Kousei could rarely participate. Being with Kousei for a long time, Tsubaki gradually took on the role of an "older sister". Kousei was that "hopeless little brother she had to take care of". She would often prepare gifts for Kousei and cheer him up when he was sad, such as when his mother abandoned his pet cat he had found and he couldn't find him. Rather than being interested in music, Tsubaki was always athletic.