*Mal. I see him as more of a reactor than a big idea person. I'd reference "The Message" and "The Train Job" for this. He didn't think of the bigger picture in either of those incidents, and some of them caused him a whole heck of a lot of trouble later on. Mal seems more introverted to me, but I could see him as an extrovert too. He's not a stupid man - he's extremely smart - but I don't see him as a big picture guy. He gets it in his head to do something and he does it, and damn the consequences.
Arguments for ENTP
ENTP Ego:
Mal: "We're not gonna die. We can't die, Bendis. You know why? Because we are so...very...pretty. We are just too pretty for God to let us die."
Mal: "It's a real burn, being right so often."
Zoe: First rule of battle, little one. Don't ever let 'em know where you are.
Mal: Whoo-hoo! I'm right here! I'm right here! You want some of me? Yeah, you do! Come on! Come on!
Zoe: Of course, there are other schools of thought...
ENTP Humour:
Mal: "I don't believe there's a power in the 'verse that can stop Kaylee from being cheerful." (he smiles at her, never stopping working) "Sometimes you just wanna duct tape her mouth and dump her in the hold for a month."
Mal: "And Kaylee, what the hell's goin' on in the engine room? Were there monkeys? Some terrifying space monkeys maybe got loose?"
Mal: How can you say that? How can you shame me in front of new people?
Jayne: If I could make you prettier, I would.
Mal: You're not the man I met a year ago.
Inara: "So, explain to me again why Zoe wasn't in the dress?"
Mal: "Tactics, woman! Needed her in the back. 'Sides, them soft cotton dresses feel kind of nice. There's a whole airflow."
Inara: "And you would know that because...?"
Mal: "You can't open the book of my life and jump in the middle. Like woman, I'm a mystery."
ENTP Abrupt and Violent Response:
Mal: "If anyone gets nosy, just...you know... shoot 'em. "
Zoe: "Shoot 'em?"
Mal: "Politely."
ENTP Loyalty and Humility(shutupwehasit):
Simon: You came for us.
Mal: You're on my crew.
Simon: Yeah, but you don't even like me. Why'd you come back?
Mal: You're on my crew. Why we still talking about this?
ENTP Ability to see something where there's nothing:
Mal: Ship like this, be with ya 'til the day you die.
Zoe: Yes Sir. Because it's a deathtrap.
Mal: That's not... you are very much lacking in imagination.
Zoe: I imagine that's so, sir.
Just pure awesome.
Mal: "Are you offering me a trade?"
Jayne: "A trade!? Hell, it's theft! This is the best damn gun made by man. It has extreme sentimental value. It's miles more worthy than what you got."
Mal: "What I got? She has a name."
Jayne: "So does this!" (caresses the gun lovingly) "I call it Vera."
Mal: "Well, my days of taking you seriously are certainly coming to a middle."
In the episode
Train Job he came up with a brilliant plan for a crime they'd never committed before, and kaylee called him a supergenius plan maker. He fits the type of an ENTP with nearly all of his creative planning, amoral illegal actions, and acerbic wit. There's no way he could be an ISFP, he's nothing like either my cousin or my sister in temperament.