The Portia depicted in Julius Caesar, Porcia Catonis, wife of Marcus Brutus, is not a chief character in the play, but she displays very strong qualities of loyalty, character, love and obedience. She is deeply in love with her husband and is offended when he, at first, refuses to confide in her about what is troubling him. To convince Brutus of her loyalty and trust, she stabs herself in the thigh, displaying rare strength and conviction. Brutus is impressed by her remarkable resolve and thus reveals to her the plot to assassinate Caesar. True to her promise, Portia never betrays Brutus.
Not only does Portia have strength of character, but she is also endowed with great beauty, gentleness and wisdom. So great is her devotion to Brutus that she commits suicide by swallowing hot coals, distraught by his absence and the fact that she might never see him again.
Portia, in The Merchant of Venice, is the heroine in the play. She is a wealthy heiress who abides to her father's instructions (in terms of his will) to find an acceptable suitor. She is so committed to her father's instructions that she follows them to the letter, although she might find herself with a husband that she does not love. Fortunately, this does not happen, for the one she loves, Bassanio, chooses the right casket.
Shakespeare's inspiration for this Portia is derived from Portia in Julius Caesar. Bassanio makes reference to her by saying:
... and she's worth no less
than Cato's daughter, Brutus' Portia.
Bassanio's reference is not misplaced because Portia here has beauty, loyalty, wit, intelligence and grace, just as did Brutus' Portia. She commits herself to her suitor's friend - Antonio, and vows to assist him in his direst need. She offers to settle Antonio's debt and later magnificently defends him in court by posing as a lawyer. Antonio is released and Shylock punished.
It is clear that the two Portia's both have the same and similar qualities, loyalty, trust, intelligence, strength of character and beauty, aptly displayed by their actions in both plays.