They are meant to be these entities from outside of our reality who are only taking on human shapes in order to do specific jobs, but the way their dynamic and their relationships with other characters is obviously informed by their genders and female and male, respectively.
Now obviously that is mostly because these are characters in a TV show played by a man and woman and written and acted as such and take on roles that you might expect from that. Sapphire is more empathetic. She understands and navigates social customs much better. She ends up needing to be rescued more often. Steel is much more blunt and direct. He is the one who takes action. They are, for the most part, essentially just a man and a woman with magic powers and a weird job who are just very distant from most people.
But taking a more Watsonian perspective it is interesting to think about how these are being with specific specialised abilities trying to solve problems. Sapphire is empathetic not in the sense that she is particularly caring of other’s feelings but in that she has perceptions not limited by time and space and simply know someone’s life as easily as she can see what colour hair they have. Part of her job and her being is to know and understand things, including navigating the human elements. Thus when she, or perhaps it, an elemental entity that exists outside of reality, is projected into the social context of the late seventies Britain her job is to be the woman.
And Steel, being the straightforward one who who can freeze things in place or block a knife with his hand, whose abilities revolve around acting and doing and resolving, plays the role of the man.
