Imma just settle the Percy fell first vs. Annabeth fell first debate once and for all:
While tessa.chb did give us admittedly glorious answer of “that’s so funny” *plops down MoA* “I seem to remember them falling at the exact same time”, I have decided to do an actual real breakdown of the Percabeth feelings timeline and give you the most scientifically/canonically accurate answer.
Annabeth says in Mark of Athena that she’d had a crush on Percy since they were 12, so Lightning Thief, but that in the past year (so, Last Olympian and on) that her feelings had grown painfully intense.
In contrast, Percy doesn’t do this kind of self reflection, but we do spend the first five books inside his head. In Lightning Thief, he’s mostly just intimidated by her and learns to see her as a friend, no signs of any romantic feelings yet. In Sea of Monsters, he really cares a lot about her, considers her pretty much his best friend, and is perhaps a bit attracted to her, but doesn’t really seem to have a full crush on her.
Enter Titan’s Curse, which I feel is definitely the turning point for Percy’s feelings for Annabeth. Here, at age 14, he’s fiercely protective of her, wants to violently murder a 10 year old for asking if she’s his girlfriend, is extremely distressed at the thought of her potentially joining the Hunters of Artemis, and admits that for a moment, when she smiles, Aphrodite looks like Annabeth. At this point, the boy is absolutely down bad. He loves her so completely that he would rather burn down the world and everyone in it than risk losing her.
In BotL, when they’re 14, almost 15, Percy and Annabeth spend all of their time together, even if they’re just doing camp chores. The entire camp ships them. They were literally going to go watch a movie together (based on my carefully designed timeline designed to limit anachronisms, I believe they were going to see the first Avengers movie), and Percy is mortified when Sally calls it a date. Annabeth’s brother literally ignores a camp rule designed to prevent demigod teenage pregnancy and lets the two of them be alone together in the Athena cabin. They disappear for what to them feels like 5 minutes and the entire camp thinks that they ditched capture the flag to make out in the woods. Both of them get really jealous when they feel like their potential future relationship is being threatened (either by Percy thinking Annabeth has a crush on Luke, or by Rachel thinking she has a crush on Percy).
Annabeth kisses Percy and he literally forgets his own name! If it weren’t for the life or death situation, he admits he probably would’ve just stood there in stunned silence trying to get his brain to reboot for a couple hours.
I forget if this is BotL or Last Olympian, but at one point they get in a big fight where Annabeth accuses Percy of running from his feelings and calls him a coward. It’s clear at this point that she’s picked up on his feelings for her, even though he hasn’t really figured them out for himself yet. (Yep, Annabeth knew Percy loved her before Percy knew he loved her.)
Percy admits that the year where they’re 15 is pretty tense between them. They both have feelings, they’re under a lot of stress, Annabeth’s pretty sure Percy’s gonna die because of the prophecy, and on top of all that, Percy’s best mortal friend is a girl who keeps flirting with him, so every time he needs a reminder that the mortal world actually exists, or just to take a break and feel normal for a while, he’s hurting Annabeth further. It’s messy, it’s complicated, and they’re 15. When they finally get together at the end of the book, it feels so right, so easy, simple and ultimately inevitable, because once all the perceived competition is gone, and the life threatening prophecy is over, there’s nothing left in their way but teenage hormones and awkwardness.
Looking at the way that Percy’s feelings for Annabeth progress throughout the original series and Heroes of Olympus, there’s honestly not much of a difference between Titan’s Curse and Mark of Athena or House of Hades, except that by then, there’s less uncertainty and complexity, so he’s able to think about them having a future together. This suggests that he more or less skipped the crush phase and his platonic love for her just evolved into being romantically in love with her.
So, based on this we have the following timeline:
12: Annabeth is crushing, Percy is not
13: Annabeth is crushing, Percy isn’t really but might be starting to?
14: Annabeth’s crushing, Percy’s in love with her but doesn’t realize it yet.
15: Annabeth’s crushing and suspects that Percy has feelings for her. Percy’s in love and is maybe starting to catch on?
16: Annabeth falls in love, Percy fully catches on to his feelings, and they get together.
So, the answer to the question of who fell first?
Annabeth crushed first, but Percy fell first.