Hiroki Kasai has performed extensively on the viola across Europe, Asia, Australia, and the United States, appearing at prominent music festivals such as Beethovenfest Bonn, Marlboro, Verbier, Pablo Casals, Schleswig-Holstein, and the Gstaad Menuhin Festival.

He recently won First Prize at the 30th International Johannes Brahms Competition in Austria, as well as Second Prize and the Audience Prize at the 6th Tokyo International Viola Competition. As a soloist, he has appeared with orchestras in Asia, Europe, and the United States, including the Tokyo Philharmonic and the Schleswig-Holstein Festival Orchestra. A passionate chamber musician, Hiroki has performed with artists such as Lawrence Power, Sayaka Shoji, and Ivan Monighetti. He has participated in Prussia Cove Open Chamber Music and the Seiji Ozawa Academy in Switzerland, and took part in an extensive string quartet tour across Japan with violinist Midori. His solo and chamber music performances have been broadcast by numerous radio stations, including NHK (Japan), RNE (Spain), NDR (Germany), RTS (Switzerland), and NPR (United States).

Hiroki graduated with First Class Honours from the Royal Academy of Music, where he studied violin under Rodney Friend. In 2022, encouraged by Nobuko Imai, he began studying the viola and completed his Master’s degree in 2025 under Ms. Imai at the Escuela Superior de Música Reina Sofía. Upon graduation, he received a diploma from Her Royal Highness Queen Sofía of Spain as the top student in his class. From the 2025–26 season, Hiroki is a scholar at the Orchestra Academy of the Bavarian Radio Symphony Orchestra in Munich, under the direction of Sir Simon Rattle.