Who moves every episode forward? Mulder. So he's the main character.
On wikipedia Mulder is called a protagonist, Scully is called a partner.
She matches most definitions of sidekick, and I feel like the objection to calling her one is just that she's a woman. If her character was instead a guy called Pater (similar play on 'skull') he'd be a sidekick.
Who moves every episode forward? Mulder. So he's the main character.
On wikipedia Mulder is called a protagonist, Scully is called a partner.
She matches most definitions of sidekick, and I feel like the objection to calling her one is just that she's a woman. If her character was instead a guy called Pater (similar play on 'skull') he'd be a sidekick.