Portia is Brutus's devoted wife. She doesn't get a whole lot of stage time but we think she's an interesting figure, especially when it comes to the play's concern with gender dynamics.
When Brutus refuses to confide in Portia, she takes issue with his secrecy: as a married couple, she says, they should have no secrets.
Dear my lord,
Make me acquainted with your cause of grief.
Within the bond of marriage, tell me, Brutus,
Is it excepted I should know no secrets
That appertain to you? Am I yourself
But, as it were, in sort or limitation,
To keep with you at meals, comfort your bed,
And talk to you sometimes? Dwell I but in the suburbs
Of your good pleasure? If it be no more,
Portia is Brutus' harlot, not his wife. (2.1.5)
In other words, Portia is sick and tired of being excluded from her husband's world just because she's a woman. She also suggests that, when Brutus keeps things from her, he's treating her like a "harlot [prostitute], not his wife."
Portia's desire to be close to her husband seems reasonable enough. But Portia also has the annoying habit of talking about women (including herself) as though they're weaker than men.
I grant I am a woman; but withal
A woman well-reputed, Cato's daughter.
Think you I am no stronger than my sex,
Being so father'd and so husbanded?
Tell me your counsels, I will not disclose 'em:
I have made strong proof of my constancy,
Giving myself a voluntary wound
Here, in the thigh: can I bear that with patience.
And not my husband's secrets? (2.1.6)
Here Portia says she knows she's just a girl, but since she's the daughter and wife of two really awesome men, that makes her better than the average woman. To prove her point, she stabs herself in the thigh without flinching and demands that her husband treat her with more respect. Yikes! Later she kills herself by swallowing "fire," or hot coals (4.3). This is interesting because it's usually men who are prone to violence in the play.
History Snack: When Portia says she knows she's just "a woman" but she also thinks she's "stronger" and more constant (i.e., steady and masculine) than most, she sounds a lot like Queen Elizabeth I (Shakespeare's monarch) who famously said "I know I have the body but of a weak and feeble woman; but I have the heart and stomach of a king" ("Speech to the Troops at Tilbury", 1588). Queen Elizabeth I, like Portia, buys into the idea that women are weaker than men but also presents herself as the exception to the rule.