SPOILERS AHEAD

Arya Stark. She starts out as a rambunctious tomboy and ends up a face-changing assassin. Even though she's one of the youngest characters, she spends very little time crying about how shitty life is and instead pursues her goal of murdering everyone who has ever crossed her.

Arya doesn't give a shit about politics, houses, thrones or anything else. For her, it's all about family and revenge.

I'd say Arya Stark is one of the most hardcore characters in the story. She single-handedly killed every male member of House Frey at a wedding. The Red Wedding was hardcore, but it took a bunch of people to pull it off. Arya did her dirt alone.

Here is a list of everyone Arya has taken out so far in the show (she's at 63 bodies now). She's putting in some serious work for House Stark and I don't think she's even a teenager yet.