Seeing this scene animated reminded me that I had many thoughts about it back in the day. I thought it might be fun to share those.
Unless I’ve missed something, it’s never explicitly stated why Jean stayed with Erwin after the Female Titan was captured in Stohess. He could’ve just as easily gone with Eren, Mikasa and Armin as they travelled to Ehrmich District with Levi and Hange. But he didn’t. He stayed with Erwin.
It’s been mentioned in the smartpass au that Erwin takes the time to learn the names of all the new recruits. Typically only a handful of trainees choose to join the Survey Corps each year, so Erwin knowing Jean’s name is nothing extraordinary.
But Erwin, brilliant mind that he is, would go far beyond knowing just the names. He’d also carefully read the assessments Keith Shadis provides. These kids are pieces on Erwin’s chess board. They are tools for him to use and maneuver, so he’d be on the lookout for new talent. In Jean’s case the assessment was “…he can take in a situation at a moment’s glance, but his personality is like a drawn sword, which tends to create conflict.”
My headcanon is that after reading the assessment, plus seeing reports about Jean from Trost and the encounter with the female titan, Erwin may have suspected leadership potential in Jean. Just as he knew Mikasa was more valuable than a 100 soldiers and Armin has a sharp and strategic mind, he may pegged Jean as a future team or squad leader. Maybe he kept Jean with him to assess and develop that potential.