In league in trickery with Portia, Bellario writes to the Duke that he is ill and unable to attend the Duke's court. He also writes that at the time of receiving the Duke's letter, he was being visited by a young lawyer named Balthasar from Rome. Bellario goes on to say that he and Balthasar discussed the case and examined may law books to find the legal precedents that apply to Shylock's case against Antonio and that Bellario sent young Balthasar to the Duke instead of making the trip himself. He also writes that Balthasar is fully equipped with Bellario's opinion on the case and has enhanced Bellario's opinion by the addition of his own learned opinion. Bellario beseeches the Duke to give Balthasar "reverend estimation" despite his youth for he is wise though young.
I am very sick: but in the instant that your messenger came, in loving visitation was with me a young doctor of Rome; his name is Balthasar. I acquainted him with the cause in controversy between the Jew and Antonio the merchant: we turned o'er many books together:
Balthasar the young legal doctor from Rome, of course, is a made up person who doesn't exist and was not visting Bellario who was not sick. Remember that Portia sent her servant--named Balthasar--to deliver the letter to her cousin Bellario in Padua with instructions that the servant--Balthasar--bring all the things that Bellario will give him and then meet her and Nerissa in Venice. The whole letter was a plot, a prank, a ploy to get Portia into the courtroom while disguised as a male lawyer--named Balthasar--with Bellario's legal instructions rehearsed and in hand, so she could win Antonio's case and save him from Shylock's avenging knife. Even though Balthasar doesn't exist, Portia was dressed as a lawyer and was masquerading in disguise as Balthasar the young lawyer.