Go to your My Documents folder, go to My Games, Elemental, and then Units.
In there you will find your custom sovereign once they're created. Open that file and you can freely alter their stats, books, starting equipment, etc if you wanna be on par with a premade. This won't affect a game in progress, if I recall correctly, but once altered, your sovereign will reflect the changes on the Sovereign select screen, so you will only have to do this once. So if you're starting a new game, create the sovereign, quit the game, make your changes, then play.