Alex doesn't have a last name. Heck, his first name might as well be Love Interest. He's a flat character and we're not just talking about his abs. His primary purpose in the book is to make Lena feel like a princess, and be that perfect romantic dude that no real-life person could ever live up to.
He guards her safety. He has a cutesy pet name for her: Lena-Loo, which sounds like a combination of her name and a British slang word for toilet. He's studly without a shirt, reads her poetry under the stars, and has the handsomest quirks ever bestowed upon a human male.
Lena says, "When he gets serious a little crease forms between his eyebrows, a tiny comma, and it's the cutest thing I've ever seen" (16.34). Okay, punctuation really lights Lena's fire. Here's what this couple's pillow talk probably sounds like: Dash. Question mark. Semi colon. Exclamation point.
On top of all that, Alex gives his life to save her. While Lena doesn't admit it outright, she does have a bit of a sacrifice fetish. That's her favorite part of Romeo and Juliet, which she confesses at the evaluation, saying, "Sacrifice. I want to say something about sacrifice, but the world doesn't come" (4.49).
As if Lena didn't love Alex enough already, him becoming a martyr to save her in the end totally clinches it. We don't think she'll ever be able to get over him. And we understand. It's hard to get over someone when they're more of an idea to you than a real person; Alex represents wildness, worldliness, and sacrifice to Lena.