Necessary Tier
3DS
Awakening game cart / digital download
Vision
Working hands
Sentience
Frederick
Pretty Good Tier
Avatar
In case you couldn't tell, I find "necessary therefore best" arguments incredibly stupid. By that logic, most lords would be at the top of top tier, as seizing is necessary to complete the game. And what would we do with healers? Healing isn't strictly necessary to complete Awakening, but offense is. Should we move them to bottom because they aren't strictly necessary?
Character rankings are based on contributions. Frederick contributes in the first four maps to an incredible degree, but what Avatar does for the rest of the game is more valuable on the whole, as it spans a longer period of time. (P.S. The first four maps can technically be solo'd with any set of Paired Up units if you get enough Dual Guard rolls, so Frederick is never completely necessary.)