Hirokuni Kasuga is a third-year student at Kanagawa High School and a member of the school's rugby club. His name carries the meanings of spring and day, as well as tolerant and generous. He was born on March 13. Kasuga is known for his tough personality and is fundamentally taciturn, often remaining silent. Despite having a scary or intense facial expression, he can be unexpectedly helpful, taking on a supportive role similar to an older brother to his teammates. His primary motivation and enjoyment in rugby comes from the physical act of tackling. This love for the tackle leads him to keep a close eye on Gion, as he acknowledges the first-year player's own competitive spirit and shared passion for tackling.

In the team lineup, Kasuga primarily plays as an openside flanker. He also has experience playing in the backs, specifically as a center, which gives him a well-rounded knowledge of the game from both forward and back perspectives. Because he only started playing as a flanker in his third year, his understanding of different positions is notable. When another player, Ise, expressed a desire to move to flanker, it was suggested that Kasuga could potentially move back to playing center. Among his teammates, he has a noted dynamic with Hyosu, who plays the same position as him but finds Kasuga annoying.