Emperors are the ultimate foot-based class in any game they appear in, have very high stats and in Genealogy of the Holy War they can use most weapons. Emperors also have capped out speed and resistance stats, an area where Generals rarely do well in.
The Fire Emblem: Binding Blade class King is noted to be similar to the Emperor class. The main difference being Emperors always can use lances in some form, while kings only use swords, however the staff form of the Eckesachs held by Zephiel, the only King, is reminiscent of a lance. The Emperor class in Fire Emblem: Shin Monshō no Nazo even looks identical to the King class of Binding Blade.