They couldn't be more different.
Sansa flaunts long, pale legs and wide, blue eyes, tosses gold-spun hair over her shoulders as she flashes smiles to everyone and anyone. Under the glow of golden, dusty pub lights she pouts strawberry lips over cocktails she didn't pay for, batts her lashes at men she's never met, lets laughter spill from her mouth like music, a perverse tune that lingers long after it dies. She delights in their lustful gazes, and why shouldn't she? This game is what she's best at.
Arya hacks at dark tresses with her knives, mauls her lip as she worries it between her teeth, sucks the metallic taste of blood from the flesh and scowls at those who dare to stare. She slips into the shadows, so tiny she's barely there as she ghosts towards her next victim, growling like a wolf in both anger and revelry at those who attempt to fight before she slips her knife between their ribs. She delights in the raw fear in their eyes, and why shouldn't she? This game is what she's best at.
They couldn't be more alike.
They do their hunting at night, stalking their prey with hunger in their eyes before ripping it from limb to limb, bathing in the screams. They get what they want with pain or pleasure, what's the difference, really? At the end of the night they both lick blood off of their teeth, pick flesh from under their fingernails, and smile.
Sometimes they wonder what they'd do without each other.
They wonder how they'd rest each night without the heat of their bodies lying side by side, wonder how they'd lull themselves into blissful sleep without the other whispering the most intimate details of their latest hunt in their ears, wonder what it feels like to be alone, to be without your shadow, your sister, your blood, without the other half of your soul.
(They try not to wonder for very long.)
They should be broken, maybe, with all the things they've seen and all the things they've done, all the family they've lost, nothing but the other left to them. Maybe they are, because sometimes when they're tangled in each other in the middle of the night, they'll feel each other shaking or sobbing quietly, oh so very quietly. But these moments don't define them, don't taint the reputations they hold in such very high esteem, but rather confirm to them what they need to know: that they're wonderfully, vividly, painstakingly alive. Lives painted in red (lipstick and blood) and lives painted in black (mascara and shadows) but alive nonetheless.
And best of all, alive together.