Tennis: Bind Longma at the beginning, double the return

A sports idiot travels to the world of Prince of Tennis. Poor talent? No problem, the multiplier return system allows you to show off, directly bind to the protagonist, and multiply the return strength. "If you can bind, why can't you bind Nanjiro?" "Ryoma, you are 9 years old and you haven't started the Thousand Hammers, I am deeply worried." Many years later, Ryoma, who left the professional arena, wrote a book...