In Act III, Scene 4 of The Merchant of Venice we learn that Doctor Bellario, a prestigious Doctor of Law, is Portia's cousin. We also learn that he is so devoted to his cousin that Portia has no doubt that whatever she asks, he will do. We are not told precisely what she asks, but we know that it has something to do with garments and information contained in notes.
In Act IV, Scene 1, we discover that the garments from Bellario are the academic robes of a Doctor of Law and a Clerk of Law. Portia wears the former (clothes identifying a Doctor of Law) and Nerissa the latter (clothes identifying a Clerk of Law). We also learn that the Duke, unbeknownst to Portia, has sent for Doctor Bellario to come and help settle the case between Shylock and Antonio. We next learn that one of the notes Balthasar (Portia's messenger to Bellario) brought from Bellario is a note of introduction addressing the Duke and begging him to accept the disguised Portia, going under the name Balthasar, as a substitute for his own presence, pleading that ill health prevents him from coming himself.
We must surmise what the other note(s) from Bellario contained. The correct surmise is that the note(s) contained instruction on how to win the case: (1) plead for mercy; (2) offer greater than the owed sum and (3) invoke the law protecting citizens of Venice from threats against their lives by non-citizens (Jews were not citizens; they were non-citizens).
It is clear that Dr Bellario is Portia 's cousin. This can be identificed by the audience when Portia tells one of her servants to go and bring the documents from Dr Bellario.
The Doctor later proves to be a very important character as it was with the identity of the Dr that Portia is able to save Antonio.