Disclaimer: I own nothing but the idea for this story and series (I hope)... X-Men and everything related to them belongs to Marvel, while Dawn and the rest of the Buffy-verse belongs to Mr. Joss Whedon. Hope they don't feel obligated to sue and you enjoy this
He's the first one to smile her way when she shows up, just the slightest hint of an interest in his 'Hey'. Dawn flushes and her eyes can't but follow him, he's gone before she swallows back the nerves to say 'Hi'.
He's her first contact with the reality of a 14-year-old teenage girl. The one girl who's not as normal as she'd probably wish she could be, and never will, but a girl all the same.
He's the first time a boy ever makes her stammer and blush. It takes him less than the arch of his eyebrows to invoke both, the merest contact with his eyes to undo her forever.
He's the first kiss she ever knows, kiss of a lover, of someone who's more than a friend. It's lips touching lips, hands reaching out for hands that're there, forever.
He's the first to cause her brother's jealous ire. On one occasion their kissing turns too public, reckless, and they're caught; he manages to evade hot red laser burns, resigns himself to count down the months.
He's first to congratulate her birthday, hands holding her waist. They kiss when she's finally 18, in front of everybody, and she swears she can taste the fire that makes him one such.