It's a bit funky how Three Houses has a small roster but they decided to not include some more route-specific, non-student units. Somehow only BL gets this privilege in form of Gilbert who is technically not even affiliated with the Kingdom anymore (as he is a Knight of Seiros). They could have done more with that.
GD has 2 obvious choices in Judith and Nader. It's very weird how Judith is not playable considering she joins your army and only hangs around the monastery. Nader is a bit of underbaked character considering his title as "the Undefeated" as you don't see much of him beyond the Alliance part on the True BE route and one chapter in GD. Also he is jacked-up Wyvern Rider, I need to break this game even more.
Another unsung hero of the Alliance is Holst. He is apparently one of the best generals in the Alliance and everyone strokes his shaft, only for him to get his butt handed to him by Nemesis and poisoned mushrooms.
BE really should have made Randolph and Ladislava playable. You don't get any perspective on their characters, even on BE, and they come off as cannon fodder at best (no joke, that's how they die on all routes). Especially Ladislava doesn't have anything beyond "I owe Edelgard for some unnamed reason" and that's pretty sad.
I guess they ran out of resources to make more support convos?