John Boyega plays Finn
We see him in a jacket and and dark shirt with pants, we also see him in a Stormtrooper uniform. Is he a trooper that has defected and is on the run, or is he disguising as a trooper?
Daisy Ridley plays Rey
There is a belief among fans that she is the daughter of Lieh and Han, therefore she would be Rey Solo or Rey Organa, but that is unconfirmed. The only costume we've seen her in is reminiscent of Luke's tunic on Tatooine, this planet were are told is Jakku.
Oscar Isaac plays Poe Dameron
Beyond being an X-Wing pilot, we don't know much about this character.