Ok. People who know about Fire Emblem. If you had to pick 4 characters to add to the rider based solely on uniqueness, who would you choose. The only one I know is Medeus.
Medeus is a pretty meh choice. I'd argue he's actually awful.
I'll give ya some suggestions.
I'll assume that mounted characters are a no go due to design issues.
Hector is an armor and wields an axe.
Ephriam wields a spear.
Micaiah and Lilina could be full mages instead of Robin's Mage Knight.
Tiki transforms completely into a dragon.
Takumi could give you an archer based moveset, along with wind powers.
Jakob and Felicia and Kaze could give you a projectile based Ninja.
Black Knight is the only FE villain that should ever be considered and he has a huge sword and heavy ass armor.
While more minor, Panne, Keaton, Kaden, Lethe, etc could all be beast characters who fight like were wolves
Ultra minor but Fates has a ton of strange classes, like the mechanist where you can create a copy of yourself.
Lots of stuff.