Jealousy
Reus is incredibly jealous when Byleth starts to emotionally open up around the students at the Officers Academy.
He tries so hard not to be. Byleth has friends! Who seem to be really good people! And frankly he likes them and would like to be friends, his interactions with his peers are limited.
Reus had learned how to be simple in gesture, to learn to read Byleth’s lack of emoting.
And now she was learning to smile with people who were strangers, was beginning to learn social cues for emoting.
He’s so incredibly happy for his twin, but he wanted to be the one to show her how to feel a smile, and not just fake it like when he tried to teach her when they were kids.
Perhaps that is how they grow up. The Eisner twins follow their own paths, rather than being joined together forever.
(He’s not sure how to feel about the revelation, but he locks away the jealousy to simmer because he doesn’t really have a right to feel that way...does he?)