After having read about how the creator of Gilligan’s Island based it on the seven deadly sins, it occurs to me that you could apply that idea to just about any show with a sufficiently large cast. I think it works pretty well, though not perfectly, with Firefly. Hear me out:
Shepherd Book is God. He is the only one not beholden in some way to Mal. He tries to offer guidance to follow the path of good, but is frequently not listened to. He is the most mysterious, has no origin story, and is almost ineffable.
Malcolm Reynolds is Satan. His name itself, “Mal”, is the Latin root for “evil”. He leads the crew into crime and temptation, and they follow. When asked by the operative what his sin was, Mal said, “Hell, I’m a fan of all seven”.
Inarra Serra is Lust. That one’s pretty easy.
Hoban “Wash” Washburn is Gluttony. The guy wears a Hawaiian shirt 24/7. You see anybody do that and you just know that they’re pounding barbecued ribs and beer when you’re not looking.
Jayne Cobb is Greed. The dude is a mercenary that would push his own grandmother down a flight of stairs for the coins in her pocket. He will shoot you down if he is getting paid.
River Tam is Sloth. Go with me on this. She is a talented genius with almost inhuman martial arts skills, but she does not lift a finger for most of the series. She is so inert that when we are first introduced to her character, she is asleep in a box.
Zoe Washburn is Wrath. Like Mal, she is still furious about the Battle of Serenity. She will never forgive the alliance, and will shoot you down even if she isn’t getting paid.
Kaylee Frye is Envy. When she’s not mooning over the good doctor, she is gushing about how interesting Inara’s lifestyle must be. She is a brilliant mechanic, but constantly wishing for something more glamorous.
Dr. Simon Tam is Pride. He comes from a rich and privileged background - and everything about him tells you so. He views himself above and apart from the rest of the crew and would not even associate with them if circumstances hadn’t forced him too. He does not even notice Kaylee’s overtures because she is too far beneath him.
I know, it doesn’t fit perfectly and I had to stretch on a few of them, but I think it works even better than the Gilligan’s Island thing.