He meets her when he's 8 and she is 6. It's weird, he and David had been best friends forever, ever since his family moved from England to Maine.
But now, Killian finds out that David will get a sister and gets really jealous. He refuses to go play at David's house after Emma arrives and sits on his bed, playing alone with his cars.
Liam sits down with him and explains him that he shouldn't feel jealous and that David will continue to be his friend but Emma didn't have anyone in the world. They were lucky, Liam and him, that mom was still with them after dad had left; but Emma wasn't that lucky and Killian should be nice to her, because Emma had a hard life.
So finally Killian goes to David's house again, carrying a gift for Emma that Liam helped him pick. She's shy at first and hesitates to grab the doll he's trying to gift her, but at some point Killian smiles at her and Emma smiles back.
She gets laughed at during recess and David and Killian kick all the boys that bullied her. They stand in front of her, tall and proud and arms crossed, chests puffing, daring the other boys to pick on her again. (Her mom scolds him in public but takes him, Emma and David for icecream that afternoon. Liam gives him a pat in the head and tells him he's proud).
They are playing pirates one day when she gets scared and he bows that he'll always protect her. He's 10 and she's 8 and she talks more now, but she still doesn't trust much. She runs when she's scared, and he finds her each time, and tells her that she has nothing to be scared of. That he will protect her. That David will protect her.
She never became popular, but by the time they are in high school she has a few friends and a life of her own. She still runs when things get scary and he still finds her at the same spot of the park he did when they were 10. He doesn't vow to protect her anymore, because she knows he will. Instead they talk about life, about getting out of Maine and live an exciting new life.
She dates Walsh when she's 15. David hates him, Killian thinks she can do better. The next year, she dates Graham. David likes him, Killian still thinks she can do better.
And then David and he are off to college and he barely sees them. Thanksgiving and Christmas and perhaps on the summer. But he picked a sailing job and spent a few summers without coming back home. He and David still talk, but it's not as it used to be.
He literally rans again into her when he's 24 and she's 22. He's been living in Boston for two years designing and building boats and she's just moved in to join the police force. She was actually arresting one of his friends for public disorder and when Killian walked out of the bar to tried to convince the officer, he realizes is Emma. And she looks beautiful and fierce as she's handcuffing his friend that he can barely talk. He finally gets his act together and tries to smooth talk her into letting his friend go, but she refuses.
He follows her to the station, he has to bail his friend out anyway, but in the road he stops to get her a hot cocoa, hoping that is still what she likes.
He spends all the time they took to bring his friend out talking to her, catching up and making small talk but he can't help feeling overwhelmed by how incredible Emma is. Now that he sees her again he can't take his eyes away from her.
When they finally bring his friend out he gives her a smirk and tells her that at the risk of being beaten up by David, he wants to ask her out.
She says yes.
They kiss on the first date and he asks for another. She says yes again.
She gets scared the next day and cancels.
He finds out from David where she lives and camps outside her apartment until she shows up. And he tells her that he knows she's scared, but there is nothing to be scared. Because he told her when they 10 that he'll always protect her and he intends to keep that promise.
They moved in together six months later. And when he marries her, his only vow is that he'll protect her.