I'll rank the characters I thought were most disappointing (first gen) on each route on Hard and above. This means more or less that they didn't seem nearly worth the effort:
Nohr:
Arthur (infinitely terrible luck and speed issues)
Nyx (can't hit shit with that skill and can't get up close due to poor defenses)
Benny (joins late with 6 spd, good luck getting pavise before he's paste)
Charlotte (joins the same time as Benny but 5 levels lower, you do the math)
Hoshido:
Rinkah (mediocre bases, mediocre strength growth, one of the lowest hp growths in the game, nonexistent res and a skill that requires to not be at full health)
Orochi (terribad spd which is death for a mage)
Hayato (comes in at level 1, get out of here kid)
Subaki (The most bizarre growths on a pegasus rider I've ever seen, don't count on getting speed any time soon)