Levi does not care - or at least, he tries not to care, but his self-control is already near tethers.
He is so much older than her, and her superior. It was inappropriate to harbor the feelings he has whenever she is around him, arms-length away, alone with her.
She is a fiercely proud, strong woman, a natural fighter, unyielding, a pillar of support. Levi has seen her steel enough times in their sparring sessions and their many exchanges of caustic quips. He also knows, though, that she has her own insecurities, when they sit and chat after their exercise routine, and she voices her worries to him, as if unburdening herself to him. A deep sense of trust has been built in between them after so many years as partners in the Survey Corps.
Levi does not believe in love, has no time for a romantic relationship, but here and now what he feels must be the closest thing to it.
He knows he is doomed the moment she grabs his cravat and crashes their lips together.
The one part of his brain thinks how long he has wanted something like this, a closer, physical connection with her, something real as proof of her affections, instead of nursing dreams in which she reciprocates this passion he holds for her.
The other part rings like a warning bell: This is wrong. She's my subordinate and half my age.
But when he gasps from the initial shock of contact and she slips her tongue into his mouth, all logic, any semblance of propriety, escapes him (not that he had much to begin with, but with her it's different). He kisses her back, aggressively, like a man drowning and she is his air.
To hell with it, Levi thinks as they stumble into his room and shut the door behind them with a lock.
All that is in his head is Mikasa, Mikasa, Mikasa...
Levi can only describe their first lovemaking as something like an explosion, not like the kind a firework makes, all pretty and sparkly, but the kind from a grenade that rips one asunder into a million pieces of flesh, brutal and - dare he say it - dirty.
They both have endured their desires for so long, Levi realizes, like two trains heading straight for each other; inevitably they collide with such a violent force that both are rendered immobile afterwards, breathless and pleasantly hazy from lust.
After a while, he finds her hand, grasping her tightly, and she curls into him.
Between the two of them, words are not spoken - they don't need to be spoken.
Now, they have truly made themselves vulnerable to each other, and Levi decides he wants to protect this new thing that has finally been born between him and Mikasa.
It's his first time feeling something so powerful, but he knows for certain now - he loves her.