When Arisu plays his first game he confesses that for the first time in his life he felt alive, without anyone knowing that Arisu actually has a hidden talent to clear all games bestowed upon him.
He felt alive. This was without anyone. This was for the first time in his life. This is what he confesses. Arisu actually has a hidden talent to clear all games bestowed upon him. This was what this anyone was knowing. Arisu plays his first game.