Only really matters in one route at the end, and basically plot wise is just “more evil than you thought he was” but his fight was a good challenge–reminiscent of the final fight in Burning Blade–specially as it was at the end of my first route.
Fleche is notable for actually having agency as a side character! She does things! Huzzah! She’s literally up this high on the list for having believable reactions to events and sort of representing the human cost of war (loss of a sibling).
Gilbert’s fine? He’s the worst of the dads on your side (Seteth, Gilbert, Jeralt) in terms of being a shitty dad, but he’s the one with the most character growth by virtue of being your logistics head in the Azure Moon route. In combat he’s basically a worse Dedue–he feels more like a “so you lost Dedue” unit then a soldier in and of himself, but you’re saddled with him regardless.
Anna is always just a fun character to see again and again. Three Houses doesn’t beat you over the head with her–she’s basically just a pleasant cameo. Helps that she sells good items.
He has a cool mask and has that nice air of mystery to him, even if he’s just an alter ego for another, more dynamic character. I also have to appreciate that I never seemed to figure out how to talk to him without at best annoying him.