I'm totally on board for him being an echo! I was just curious if you guys have tried coming up with any non-echo movesets for him? I'm not too knowledgeable on Fire Emblem so I dunno what he can do exactly. I just like the Black Knight Archetype/ think their needs to be more villains.
As a non-echo, his character would be based on his class, his animations, and maybe some key abilities of his.
In the games, he's either a general, or his own class based on generals. (So you can consider him of the general class the same way Ike is of the mercenary class). His animations are particularly slow. He walks up to you, bring his sword up, then slashes at you with a destructive strike. As a non-echo, he'd need to be slower than Ike. Honestly, there's a big chance he'd become the slowest or heaviest character in the cast if he makes it in as an original. I even think these discrepancy between his and Ike's speed, weight and build are the most pertinent counter-argument to him being an echo.
He has a blessed armor, making him effectively immune to normal attacks; that's one of the reason Ike brandishes Ragnell, as it's one of the few weapons able to damage him. That blessing would probably translate into him having super armor in a lot of moves, maybe even passive super armor like Bowser.
His signature move is Luna, which is a descending strike that divide defence by two. Do note that other units may learn that technique, unlike Ike's Aether, but it's a move tied to the general class he'd represent so it's still fitting. Do also note that Aether is the combination of Sol and Luna, so the descending slash of both Ike's and Chrom's Aether is actually Luna. It'd probably look like that. (I'd say we're sleeping on this move potential as an up-B. Honestly he could just have his own "Aether" with an emphasis on the descending slash, brand it as "Luna" and that's it.)
He also uses Warp powder to teleport around the continent, hence the popular suggestion for his up-B. However, he doesn't use it mid-fight.