The Duke is, well, the Duke of Vienna. At the play's beginning, he decides to fake a vacation and puts Angelo in charge of handing out justice.
Some learning guides will tell you that Duke Vincentio is a really good guy with a heart of gold. It's true the Duke loves his people and can't bring himself to punish them when they break the law. It's also true that he tries to be helpful.
Yet, we also want to point out that Duke Vincentio does some things that are a little suspect. First, he puts in charge a man who he thinks might not be up for the task. Then, he runs around impersonating a friar so he can spy on Angelo and his people. Did we mention how the Duke actually takes confession from unwitting people who think they're confiding in a friar? To make matters worse, the Duke lies to Isabella and tells her that her brother is dead, which seems pretty cruel, don't you think?
So, what's this guy's problem? Does Vincentio have some sort of a God complex? It sure seems that way to us. Is he a good ruler? We'll leave that to you to decide.
Over the years, a lot of comparisons have been made between the Duke and King James I, who sat on the throne when Shakespeare wrote Measure for Measure. Scholar Marjorie Garber points out that, for many literary critics, Shakespeare's portrayal of Duke Vincentio seems to be modeled after James' style of government, his tendency to spy on his subjects, his aversion to the limelight, and his deep religious devotion.