Maybe it's the way she smiles at him.
The apples of her rosy cheeks puff out, her beautiful turquoise eyes crinkle, and the corners of her perfect mouth turn up, and the look she gives him as she reveals her gorgeous porcelain teeth is just for him and no one else, and it makes him melt every time.
After their first kiss, the smile she gives him as she bats her eyelashes turns him to warm butter and he finds it difficult to breathe or swallow or speak for at least the next fifteen minutes while she prances around him, chattering animatedly about all the features of his new vehicle.
When they take Kristoff's new sled for a spin shortly after, Kristoff reluctantly gives Anna the reins, and she flashes the ice master an impish grin that makes him immediately regret his decision, although he can't deny that he loves her for it regardless. Sven is more than happy to oblige when she cries for him to go faster.
Maybe it's the way she says his name.
She says it in a way that no one's ever said it before, stressing the last syllable rather than the first, her voice inflecting on the end like a musical note. Sometimes she's annoyed, sometimes she's crying, and sometimes she's laughing, but any and every way, shape, or form that his name leaves her lips as she gives it life sounds absolutely flawless to him.
When Elsa announces in September that she has given Kristoff his own room in the palace for his extended stays in Arendelle proper ("We have more than enough room to spare, and you are technically a member of the royal staff now.") Anna shakes his arm excitedly and gasps his name with a thrill so ethereal it fries his nerve endings in the best way possible: "Kristoff."
When Anna innocently points out that his room is only just down the hall from her own chambers, the way she says his name is lighthearted and whimsical ("Kristoff! Look!"), as though she's already planning all the late-night castle shenanigans they can get up to together now that he won't be sleeping in the stables with Sven, and Kristoff raises a questioning eyebrow at the queen, who only offers him a shrug and a smirk.
Maybe it's the way she pushes him outside his comfort zone.
She has a zeal for life that he's never experienced in another person. Every new endeavor, every situation is like an adventure to her, one that she dives into headfirst with a passion that far surpasses his own, dragging him along with her and forcing him to truly enjoy living for perhaps the first time in his twenty-two years on this earth. He can't really blame her, having spent most of her adolescence and teenage years missing out on everything the world has to offer.
When the first (natural) snow falls the winter following The Great Thaw, it's much too early on a cold November morning and Kristoff is sound asleep in his bed until he's violently shaken awake by petite hands on his shoulders.
"Kristoff," she hisses, her voice a whisper that barely contains her excitement. "Kriiisssstoooofff."
The ice master cracks one eye open to see Anna beaming down at him in the darkness of the room, her eyes glowing and hair mussed from sleep.
"Anna? Whatimeizzit?" he mumbles in a slur, squeezing his eyes shut as he yawns.
"I dunno, like four-ish?" Anna shrugs as though the information is unimportant.
Maybe, he thinks, if he pretends to fall back asleep, she'll leave. After a few moments she's shaking him again. He should've known she wouldn't be discouraged so easily. It makes his heart beat a little faster.
"Alright, alright, what do you want?" he groans with a sob as he sits up in the bed, sounding more like a cranky child who's been commanded to do chores than a grown man simply trying to get his beauty rest. But he can't really be angry with her, anyway.
His princess gives him one of her infamous grins that spells trouble. "Do you wanna build a snowman?"
When she finds out that his birthday is in the spring, and only a month before her own, she is ecstatic.
"Kristoff!" She cries, throwing her hands up at the breakfast table. Olaf, sitting beside her, mimics the gesture without really knowing why. "We have to throw you a party."
"We don't have to do anything."
Anna pffts and swats at him with her hand. Olaf mimics this as well.
"Nonsense, Kristofer."
She's only been allowed to plan and execute precisely one event before: her own sister's birthday party the previous winter (which sort of also doubled as the official royal Christmas gala), but the experience apparently left her no worse for wear and she plunges straight into picking centerpieces and tablecloths and bunting colors and flower arrangements and everything else needed for Kristoff's birthday celebration with an amount of energy and exuberance that makes him dizzy as he tries to keep up.
At least she has the forethought to consider asking him what he'd like to have to drink at the party.
"Beer?" The princess squints her eyes and wrinkles her nose. "Beer is so bitter. Why would you want beer?"
"You don't drink it for the taste," he laughs. "Well, sometimes you do. If you get the good stuff."
"The good stuff," Anna mumbles aloud to herself, considering the idea. She taps her pen thoughtfully on her chin. Then she shrugs and scribbles something down in her notepad. "Alright. I'm fine with it, since it's what you want. And whatever my ice master wants, my ice master gets."
My ice master. It makes Kristoff grin like an idiot for the rest of the day.
Maybe it's the way she kisses him.
He very much enjoys their kisses. After nearly a year of practice Kristoff thinks he's gotten pretty good at kissing his princess, a skill that he now prides himself in along with ice harvesting, knot-tying, and his impeccable knack for reindeer ventriloquism.
It takes him an entire week after their first kiss to even work up the courage to kiss her again. She's in the stables, brushing her white mare in preparation for an afternoon ride, when he waltzes in as nonchalantly as possible after unloading a delivery of ice. Her eyes sparkle when she sees him, greeting him with a call of his name and a wave of her hand, which he grabs with his own and uses for leverage to pull the princess closer to him, landing his mouth on hers with a brief, but firm, touch. He's a blushing, blubbering mess afterwards, but it was worth it to see her smile and feel the hundreds of butterflies in his stomach take flight.
The next time they kiss, it's Anna who initiates it. He has just told her that he'll be gone for a few days harvesting ice when she pushes herself up onto the tips of her toes to give him a parting peck on the lips.
Not long after he returns, he moves into his new room in the castle, and he and Anna fall into a comfortable routine consisting of one kiss in the hall before breakfast and one kiss in the hall after dinner, with the occasional daytime kiss on special occasions thrown in for good measure.
But they've never gone farther than just kisses. Always a delicate, chaste pressing of lips, often followed by the awkward, uncoordinated bumping of noses and a smile or a blush from either one or both of them. Always proper, never lewd or imprudent, but as the months go by and his feelings for the princess become stronger and more intense with each passing day, he's starting to realize that he needs more; he needs her.
He realized around Christmas time that he doesn't just love Anna, but he's actually in love with her.
She was a vision in her crimson holiday gown when she kissed him beneath the mistletoe, as chastely as always, but something about the light in her eyes and the sweep of her hair and the easiness in her shoulders and the way she wished him a "Merry Christmas, Kristoff" when she pulled away awakened something inside Kristoff and made him feel as though he was floating clear above the ground.
The feeling is foreign to him, yet it feels as though it's been slumbering inside of him all along. At first he can't really put a finger on it, having long since forgotten what love is even remotely supposed to feel like, but apparently it's composed of fluttery abdominal muscles, heart palpitations, weak knees, and a bit of nausea whenever she's around. It also frequently results in him following Anna about the castle like a sick puppy dog, desperate for any attention from his master. He finds that he misses her often, too, whether she's a hundred miles away or right down the hall sleeping in her own room.
Perhaps the best and worst symptom of all is that it makes him crave physical affection from his princess.
The feeling overwhelms him for months until, one evening after dinner in late May, Anna reaches up on her tiptoes to give him their customary goodnight kiss before retiring to their separate rooms in the castle for the night, and Kristoff finds that he can't hold back anymore.
Anna had spent a good portion of the meal talking excitedly about God-knows-what, as Kristoff had been too enraptured in the sound of her voice and the curve of her throat to be bothered to actually process anything she was saying. His reverie is only broken when he hears Elsa cough quietly in his direction, and it's then that he realizes he's been watching Anna's animated facial expressions and hand gestures with a dumbstruck look plastered across his own features. He'd even been drooling a little, evident by the tiny puddle on the tabletop before him. Luckily, Anna hadn't noticed, and continued uninterrupted with her story while the queen hid a knowing smile behind her hand.
When Anna's lips touch his in the hallway that night, light and silky against his own chapped skin like always, he commands her mouth with his own immediately and pulls her flush against him, startling her. This elicits a squeak from the princess and she tenses in his arms, but doesn't resist, and within seconds she's experimentally kissing him back just as forcefully. Her slender arms snake around his neck as his hands find her waist, moving around to press on the small of her back, pushing their bodies closer together. He presses his tongue against the seam of her lips and she cautiously opens for him, allowing him access to the inside of her mouth, previously uncharted. She tastes sweet, like a mix of cane sugar and mead and nutmeg, and the flavor makes Kristoff's head spin. He takes his time exploring her as her own delicate tongue dances carefully over his. The kiss is moist, and sloppy, as they're both wholly inexperienced in this particular department, but Anna involuntarily moans anyway and Kristoff feels a familiar twinge in his pants at the sound.
When she pulls away first, her freckled face is flushed and her chest heaves with the effort of catching her breath; her pink tongue darts out to lick her swollen lips, tasting the remnants of their kiss, tasting him on her, her blue eyes twinkling with some new life in the dim light of the hallway as she stares at him. Her fingertips come up, curiously, and touch her mouth, as if she can still feel his phantom lips on hers. It's as if she had never considered that they could take their kisses further, that kissing so passionately and with tongues was even an option on the table, and to Kristoff that's so much hotter than if she had known how great kissing could be prior to this.
He thinks briefly of how easy it would be to throw the Crown Princess over his shoulder and carry her to his room. He imagines what her bare breasts would feel like under his hands, how her soft stomach and the curve where her thigh meets her hip would feel beneath his lips, how beautiful she'd look lying naked on his duvet.
He's never wanted any woman as badly as he wants Anna.
Maybe it's the way she smiles at him in that moment and bids him goodnight, before turning on her heel and heading to her room with a little extra skip in her step, that Kristoff knows without a doubt: he's desperately, madly, head-over-heels in love with her, and he needs to find a way to tell her before the emotion eats him alive from the inside out.