The issue of "Jack's Twin" is only, as Luna states, "...Leo being stupid." This is not a Side-Quest as what Leo says is not spoken in red font. Only when a character or object uses the red font does it indicate that it is either important or it is a task that must be completed. But you can do something with this. If you're playing as a boy you need to make Jack your tag partner (beat him 3 times then go to bootleg to make him your partner), change into King replica (beat jack 10 times in a normal duel and a turbo duel) then talk to Leo again then you get vice dragon. If you're a girl you still get vice dragon but you need to make akiza your tag partner and wear the witch's dress.