Train Heartnet A.K.A. number 13 of the secret society Chronos, is known as the Black Cat thanks to his quick reaction times when engaged in combat. He is an unbeatable master gunman and a cold and unfeeling killer until he meets Saya, a bounty hunter, or "sweeper".
She teaches him the value of a human life, that one should live to repent for one's mistakes rather than die because of them. Through her, Train learns to love and care for others as he too becomes a "sweeper" and travels with his partner Sven and the mysterious girl Eve.