If anyone needs help with recruiting units or Fire Emblem: Three Houses in general, I’m here.
To recruit units in this game, you need to level up your stats and weapon levels.
Edelgard and Hoobirb: only available on the Black Eagles route.
Caspar: Strength, Brawl
Dorothea: Charm, Authority
Linhardt: Magic, Reason
Petra: Dexterity (Dex), Riding
Ferdinand: Dex, Heavy Armor
Bernadetta: Strength, Bow
Dimitri and Dedue: only available on the Blue Lions route.
Ashe: Charm, Lance
Felix (Ablix): Speed, Sword
Ingrid: Dex, Flying
Sylvain: Charm, Reason (you can automatically get him if you're female byleth)
Mercedes: Magic, Bow
Annette (not that it matters): Magic, Faith
Claude: only available on the Golden Deer route.
Hilda: Charm, Axe (can't be recruited on the Black Eagles route cuz Hilda's not a communist)
Lorenz: Charm, Reason
Marianne: Magic, Riding
Lysithea: Magic, Faith
Leonie: Strength, Lance
Raphael: Strength, Heavy Armor
Ignatz (not that it matters): Dex, Authority
-----------Church of Seiros:------------
Flayn: joins in October
Seteth (he's kinda broken): joins in March
Gilbert: joins during Azure Moon route