He'll never be a prince charming.
He's all sharp edges and rough surfaces: harsh words, turbulent emotions, unspoken confessions. He never admits what he's feeling and despite his supposedly genius-level intellect, he's a complete idiot when it comes to emotions and psychology, and he has the sensitivity roughly equivalent to that of a blunt axe.
He's antagonistic and emotionally constipated and utterly uncharming, and sometimes Mikan wonders why she fell in love with him in the first place.
But then, right at that moment, he does something that makes her fall in love with him all over again.
The night after their first major fight, Mikan finds a box of Fluff Puffs on her bed, and a single white lily, perfect and beautiful. There isn't a note, and he'd probably meant to get a white tulip and messed up - but it brings a smile to her face nevertheless. He doesn't smile at her the morning after, but she offers him a Fluff Puff and after breakfast he pulls her into an empty classroom and kisses her, quick and hot and intense: flash and burn, just like him - and they spend the morning ditching Math together.
Another time, she gets into a spot of trouble with the Special Ability class, and is temporarily demoted to being a No-Star. He sees her sheepish look and the empty space at her collar, and without a word, he dumps half his food onto her plate, rolling his eyes when she protests that it was against the rules: "Just eat, Polka Dot. It'll be a headache if you starve to death, I don't have the time to go around attending funerals." Later, when he gets demoted as well for encouraging her misbehaviour, they sneak into the kitchen and make fried bacon and sandwiches even though it's midnight, and it's the best meal Mikan's had ever since she'd entered the Academy.
It's never a big thing - Natsume was never the type for big, dramatic proclamations of love. It's always the little things: the stack of notes left by her bedside when she falls sick, painstakingly written out in his precise hand; how he spends the evenings watching romantic comedies with her on TV, even though he hates them with a passion; the achingly tender kisses he places on her forehead, light and hesitant, when he thinks she's asleep. It's the quiet, earnest way he shows that he cares; the way he loves without words.
He's antagonistic and emotionally constipated and utterly uncharming. He'll never be a prince charming, and Mikan doesn't mind.