The English phrase 'main character' translates into French as 'personnage principal'.
The French word for 'main' is 'principal' which is of course where we get our word 'principal' from. The English 'character' directly translates as 'caractere'. Due to how nouns sometimes operate in French however, when the two words are placed together into a phrase, they become 'personnage principal.'
Alternative Words
Instead of 'main', similar words you could use include 'chief' ('chef' in French), 'leading' ('en tete'), 'first' ('premiere') or 'primary' ('primaire').
Alternatives to 'character' are 'personality' ('personnalite'), 'individual' ('individuel') or portrayal ('representation'). The word 'role' is the same in both languages.
If you're interested in learning French, why not start off with this instructional film: