I believe it was always the SnakeEyes/Scarlett thing. they used Duke because he has a speaking role and Snakes would not at that time be able to. Also Duke was the Quote"typical male" of that time frame. Tall, blond hair and blue eyes and a leader where kids could relate to him rather then a non speaking role of Snakes.
By reading you could always tell there was something between Snakes and her but in the cartoon world words speak louder then actions and I believe this is the idea they went with.