Jealous Armin fluffy headcanon
Busy with work, Armin and Annie are spread thin and are barely able to see each other during the day. At one point, Armin accidentally hears a couple of the staff they work with chatting that: oh, I saw miss Leonhart walking around a small bundle of flowers earlier in the day, wonder who gave them to her, must be a secret admirer, I'm not surprised she's so pretty, etc, etc
Armin is stuck between a rock and a hard place, busy with critical work but also feeling terrible that someone else is comfortable enough giving Annie flowers. Not only that, she appreciates the gift enough to accept and keep them. He spends the rest of the day wondering and straight up doubting every other person he meets whether they're Annie's secret admirer
At one point, he manages to calm down and convince himself that Annie probably accepted the flowers just to not be rude. After all, their work depends very much on reputation and likability
Then comes the end of the day, they finally get to be in the same room for dinner and... Annie's still got the little bundle of flowers. In fact, she's holding it with great care and it's not even a proper bouquet, but a few blossoming wild flowers wrapped in small pieces of paper, even worse, she's blushing and refusing to look him in the eye
Then she offers him the bundle and up close they look familiar. They'd passed by a field a few weeks earlier but none of the flowers were in bloom just yet and Armin had made a comment about wanting to see them and how beautiful they'd be. But since he's been taken up by work, he was going to miss the bloom for the whole year, but Annie decided to get him at least a few so he can see what they look like
He doesn't tell her, but that night he carefully presses the flowers between the pages of heavy books to preserve them and occasionally checks on them whenever he wants to remember this gesture