Kit, the Traveler will the first Side Story you will do in the game and there isn't any set location for it. Whoever you pick for your very first character, upon completing that character's first chapter, you will find Kit right outside of their starting city. Whenever you do find Kit (the game will point him out to you), walk up to him and he will ask for a Healing Grape. Hand the item over to him and the Side Story will be complete. It's as simple as that.