Ever since she leaves home in season one, Arya Stark has been steadily making her way to being a top-level badass. Her one hangup is feeling like an outsider because of her love for unladylike hobbies, but it's her love for sword-fighting that becomes her salvation. After her older sister becomes a prisoner in King's Landing, Arya begins her travels to find safety with her family and, later, to gain the skills to avenge them.
Her quest for power leads her to the Faceless Men, who give her the secrets and abilities that turn her into the small killing machine she is now. When she finally returns home to Winterfell, she's remarkably different from the child she used to be. Those around her either find her intriguing or frightening, and sometimes it's a little of both (we see you, Gendry).
"The Long Night" shows how complex Arya is as a character: she's lethally skilled in the art of battle — even Daavos has to pause his escape to admire her work. But she's not invincible, which we see when she's thrown off course and injured. Her attempts to keep off the wights' radar in the library show how smart she is, and — despite her boasting about looking forward to seeing another face of death — she's visibly scared for her life while running from the horde of wights. It's the first time we've seen Arya scared in a long time, and it's humanizing. Considering how she spends the night before battle, we can assume that being surrounded by the people she loves has reminded the assassin that there's more to life than just revenge, and now she wants to live.
But being scared has never stopped a Stark woman before, and it doesn't stop the youngest, either. After getting a cryptic pep talk from Melisandre that basically reminds Arya that she's still that b*tch, Arya runs off and proves exactly that! The littlest Stark creeps up behind the Night King and, with a sleight-of-hand trick, takes him out with a catspaw dagger straight into the belly. A whole damn legend.