The first time it happens, they're in a cheap motel room overseas after a mission that almost kills them both.
She's shaking with adrenaline, and so is he. An accidental touch sets off the firestorm that's been threatening since they day they met, and they both get swept away by it.
She's fiercer than he expects, taking over and riding them both into oblivion, but the fierce warrior fades back into the scared girl immediately afterward as the guilt sets in.
They haven't done anything wrong, but he understands what she's feeling all too well. He dries her tears and takes her out for breakfast the next morning to show her that everything is okay.
They never mention it to anyone.
The second time it happens is his fault, and should never have gone down the way it did.
He's pissed, she's pissed too - though not at him - and the rush of anger as they spar sets off another firestorm the instant he has her pinned under him.
He feels terrible afterward, even though she's fine with what just happened. After everything she's been through, she deserves way better than rough, angry sex on the dojo floor where anyone can walk in on them.
Nikita thinks so, too, and kicks his ass when she finds out. He accepts it as his just penance - she's not unduly hard on him, considering - but Nikita's words stick with him and he starts thinking about what it is he really wants.
The third time it happens, they finally get it right.
He asks her out on a date - a real date - even if the venue is a little lowbrow for an unbelievably wealthy heiress.
They go to his favorite pizza place to shoot some pool and catch a little of the big game - of course she catches everyone's eye, and of course he's a little smug about it. Neither of them really even pays any attention to either game.
It's her idea to skip the late movie and go back to her place instead - not that he's going to argue.
He makes her breakfast in bed in the morning, and wisely refrains from mentioning how adorable she looks just waking up. Nikita sees right through their (admittedly pathetic) attempts to pretend they're not still together when they call to check in - she laughs at them, Birkhoff snickering in the background the whole time, and tells them she'll call if anything comes up.
They stay in bed all day, watching action movies and ordering takeout...