Tsubaki is Kousei's childhood friend. She 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, she usually ended in failure. Being with Kousei for a long time, Tsubaki gradually took on the role as an "older sister." Kousei was that "useless little brother she just had to take care of." Rather than the music type, Tsubaki was the athletic type. She was on the softball team in both middle school and high school along with her senpai from middle school where she enjoyed the sport a lot.