Gender selection is an option that is given to the player when making a world. It allows the player to choose wether they want their player to look like a male or a female. Even if a skin mod is used, then the character's gender will still be on file.
If the player chooses to be a male, then they will have the default skin and Stevie will be their companion.
If the player chooses to be a female, then they will play as Stevie and Steve will be their companion.