You could always put in Al, Gant, and Tina from hasha no tsurugi since they are barely mentioned by history so it wouldn't take much to disguise them.
And don't forget Ike disappears at the end of radiant dawn, but he'd be harder to disguise
I honestly haven't checked the dlc characters but that makes sense. I just remember looking at growths when the game was being datamined and Conrad had an absurd res growth compared to the others.
This game also has fairly low growth rates compared to Awakening or Fates and most characters have below 5% RES growth, with the highest being Conrad at (I think) 8%
It was every fourth turn in the original gaiden so for a while it confused me too.
Then I attacked on the 4th turn and nothing happened so I just threw a bunch of units at him cuz why not, I have the turnwheel, and figured it out from there.