Here's a drabble featuring a cocky Jean, and jealousy. Originally written for a prompt from tumblr.
It wasn't unusual for Jean to be jealous. In fact, according to Armin, Jean was jealous quite often. He had denied it at first, but it seemed like the more he did, the more Armin would point out every instance that he displayed it, to the point where Jean could no longer refute it while keeping dignity.
As a trainee, he had been jealous of Eren's closeness to Mikasa. If he was being completely honest with himself (which he always was), then he still was on some level. At the time of their graduation, he had gotten jealous that Eren was a rank higher than him. After the incident at Trost - after he had lost Marco - the jealousy had intensified; he had been jealous that someone such as Eren could even have friends. He didn't deserve one, let alone two.
Perhaps it was just easier to say that Jean was jealous of Eren.
Of course, he never would say that.
As far as Jean was concerned, it wasn't a matter of jealousy. It was just that Eren took everything he loved and corrupted it. He always had to be just one step ahead of him; just one level higher than him. It had always been his fault.
But at this moment, Jean had finally achieved something that Eren never would: a relationship with Armin. He would make sure that Eren would never best him at that.
So as the daggers glared through his body, Jean couldn't help but be happy and smile; smile for the fact that, for once, he was the cause of jealousy.