Never seen the show either, but I think I saw clips of how how incredibly friendly and close they are, so I'm sure you could find a compilation of cute moments between them if your okay with spoilers
Edelgard: You have invested more in that videogame than you ever have in this relationship.
Byleth: Hey, I didn't say anything when you spent three months on end maining me in Smash netplay.
Together: It's so difficult being famous.