Robert Kidsman
Robert keeps his mouth shut for the most part, and keeps close to Peter. He's visibly uncomfortable, and clearly out of his depth, but seems grimly determined to carry out the plan.
He's clearly wary about the bit where he's supposed to try to stake Cassius, and 'make it look convincing'.
And the bit where Bertie is supposed to arrest him, only to let him go so he can freely track Jean after.