Pisanio is Posthumus’s servant, a loyal man who acts in his master’s best interests even when commanded otherwise.
He remains behind in Britain to serve Imogen and serve as a go-between with Posthumus while he is in Rome. He is horrified at being ordered to murder Imogen, not least because this means Posthumus thinks him capable of being a murderer. He therefore comes up with the idea of dressing Imogen as a man and sending her all on her own to become a servant to the Romans, her father’s enemies, in the hopes that she may perhaps manage to make her way to Posthumus. When interrogated as to Imogen’s whereabouts, he sends Cloten off to the mountains to seek her, indirectly sending him to his death. He does not think to mistrust the Queen’s assurance that the box he takes from her contains medicine.