Nearly every FE has one prepromote in the starting lineup because without them, you'd likely be dead. I know that Frederick's saved your bum before, right?

Examples:
Awakening: Frederick (Great Knight)
Shadow Dragon: Jagen/Jeigan (however you want to spell it) (Paladin)
Radiant Dawn: Sothe (Rogue [Trickster is Awakening's equivalent])
Path of Radiance: Titania (Paladin)
Sacred Stones: Seth (Paladin)

And that's not even half of them.