who is the protagonist in this play and what is he doing when we first see him?

The protagonist is 8th Juror. When we first meet him, 8th Juror is the only one in the room who has voted not-guilty, and he is defending his stance.

THREE. Somebody’s in left field. (To EIGHT) You think he’s not guilty?
EIGHT (quietly). I don’t know.
THREE. I never saw a guiltier man in my life. You sat right in court and heard the same thing I did. The man’s a dangerous killer. You could see it.
EIGHT. He’s nineteen years old.
THREE. That’s old enough. He knifed his own father. Four inches into the chest. An innocent nineteen-year-old kid. They proved it a dozen different ways. Do you want me to list them?
EIGHT. No.
TEN (to EIGHT). Well, do you believe his story?
EIGHT. I don’t know whether I believe it or not. Maybe I don’t.
SEVEN. So what’d you vote not guilty for?
EIGHT. There were eleven votes for guilty. It’s not so easy for me to raise my hand and send a boy off to die without talking about it first.