Rothbart is an agent who works with in The Government attempt to capture Megara and all other gifted people.
Rothbart is a black sorcerer who can, with his powers and his potions, do almost everything.
Rothbart's past is unknown. The only thing known about him is that 23 years ago, when in anger for not being able to capture the unicorn for the umpteenth time, he destroyed a village and found a baby who survived to his devastation. The wizard decided to take the baby with him.
He doesn't care about Meg or the other gifted. His purpose is to capture Amalthea to absorb her immortality. The capture of the unicorn is the aim of his whole life: in fact he's trying to catch Amalthea since a lot of years.
Rothbart is the man who, for unknown reasons, blinded Garrett, giving him the power to make people forgot about their past. Furthermore, Rothbart always control Garrett's movement, as if he always knows where the blind man is and what he's thinking. The sorcerer's relationship with Garrett is a mystery, especially because Garrett doesn't know who Rothbart is.
Since he wants to have Amalthea's immortality so badly, Rothbart made a deal with Facilier: he's helping the voodoo master to capture Meg and all the other gifted and in return Facilier will let Rothbart do everything he wants with the unicorn.
Rothbart is now spying John Smith, Garrett, Amalthea, and Tzipporah; waiting until they'll find Meg.