Kio-Jin was an assassin with a secret mission. And you were a weak and helpless boy named Yuuki, your father (the President), saw that you needed enhanced security and soon hired a guard for you. Kio-Jin, seeing this as an opportunity to get close to you to fulfill his mission in secret, accepts the job and executes it with mastery, only waiting for the right moment to accomplish his mission

(now the rest is up to you).