
Friar Peter appears later than Friar Thomas and serves a parallel but more active function, assisting the Duke's return to Vienna in his own identity by gathering Isabella and Mariana for the public accusation staged at the city gates. His instructions to the two women - where to stand, when to speak, how to press their case - show him operating as a practical stage manager for the Duke's carefully engineered reckoning.
A friar who helps orchestrate the public accusation of Angelo in the play's final scene, coordinating Isabella and Mariana's testimony at the Duke's direction.
In the final scene, Friar Peter also testifies on the disguised Duke's behalf when Lucio and others accuse the 'friar' of slandering Vienna's true ruler, defending a man whose real identity he may or may not fully know at that point in the scene's unfolding deception.
Friar Peter is motivated by obedience to what he believes is a legitimate religious and civic duty, assisting in a plan whose full scope - that the disguised friar he serves is the Duke himself - he may not entirely grasp until it is revealed.
Friar Peter functions as the Duke's on-the-ground coordinator in the play's final movement, managing the practical details of the public accusation with the same quiet competence Friar Thomas shows earlier, in service of a plan larger than either friar fully controls.