There's a lot of spoilery stuff with her, so simpy put, she's badass. While she's in a similar situation as Chrom, unlike him she does have a ton of unique stuff to pull from. Dual swords + dual guns and cool animations that lend themselves well for Smash Bros. Cross could get in, but the game, marketing, etc. pushes Elma much harder than Cross, and Cross's accessibility to every weapon leaves them without a personal set of weapons.
A quick example I remembered, been a while since I've played: She interrupts a "bad guy monologue" by shooting their face, even if they are in a giant robot