who is the main caracter

It is tempting to say that Juror #3 is the main character, but he is kind of a "one-note" player who keeps simply digging in his heels about the guilt of the accused.  Juror #8 is certainly the one who, by his insistence of making sure there is no "reasonable doubt," seems to run the show and keep the jurors interacting with one another.  And even though the accused is barely seen, he is a hovering presence throughout the entire play because of the stereotypical way in which many of the jurors see him.

I guess I would say Juror #8, but I think an argument can be made for other characters.