Vortigern Mimkin
I like characters with beliefs and conviction, especially when I can see the reasoning behind those beliefs. Testing those convictions against different scenarios, pressures, and consequences, then seeing whether the character changes, deepens, hardens, or breaks, can work very well for a roleplaying character meant to be your "main."
In general, a compelling character is one who feels humanized in some way, shape, or form. If you intend to play villainy through the lens of an Imperial, then the question leans more toward what makes a villain compelling rather than simply what makes a character strong.
Ursula, Scar, and Frollo from disney movies work so well as villains because they are not evil in a hollow, decorative way. Each of them has a clear desire, a personal philosophy, and a method of control.
Ursula wants power, but she gains it through contracts, temptation, and exploiting what people already ache for.
Scar wants the throne, but his villainy grows out of envy, entitlement, and the belief that he was denied what should have been his.
Frollo is frightening because he believes himself righteous; he wraps cruelty in law, faith, and moral superiority until he can no longer see the monster in his own reflection.
That is what makes them memorable. They do not simply do bad things to do bad things; they justify those actions to themselves. Their beliefs shape their behavior, their behavior harms others, and the story forces those beliefs into conflict with the people they try to control.
For an Imperial character, that same idea can be
very useful. What do they believe order protects people from? What cruelty do they excuse as necessary? What line do they think they would never cross… and what happens when the story brings them to it?