The countess is Bertram's mother and Helen's guardian (sort of like a foster mother). She's also a widow, but Shakespeare makes it pretty clear that her most important job in this play is being a mom.
In fact, some literary critics think the countess puts the mother in smother. Think about it. The countess is always up in her son's business and basically helps Helen get hitched to Bertram. When she hears about Helen's plan to marry her son, she says "Why, Helen, thou shalt have my leave and love [...] What I can help thee to thou shalt not miss" (1.3.268, 273). Later, when the countess finds out that Helen and Bertram just got hitched and that Helen is returning to Roussillon, she says "It hath happened all as I would have had it, / save that he comes not along with her"(3.2.1-2). Yikes! She might as well rub her hands together and proceed with her best evil cackle. The Countess puts Helen's happiness before her own son's, which seems like a pretty odd thing for a mom to do.
This is why feminist Shakespeare scholar Janet Adelman writes about the countess as one of Shakespeare's "suffocating mothers." Adelman points out that, at the play's beginning, Bertram is excited about leaving the nest and becoming a man at the king's court in Paris. Then, as the play progresses, Bertram's mom totally schemes with Helen. What’s the end result? Bertram ends right back at home with his mommy (and his new wife) in Roussillon. (Source).
That said, the countess isn't all bad, especially when we compare her to some of Shakespeare's other moms. Unlike Hamlet's famous mom (Gertrude), the countess doesn't run out and marry her son's uncle right after her husband dies. Plus, it's not like she goes around talking about bashing in the heads of babies like Lady Macbeth. Remember her? She spends all her time praying that some "murderous spirits" will "unsex" her so she won't ever be mistaken for somebody's mother.
Our point? The countess isn't perfect, but she's no Lady Macbeth either.
That said, the countess is also considered one of Shakespeare's strongest female roles. (That's why the part has attracted award-winning actresses like Dame Judi Dench and Peggy Ashcroft.) In Shakespeare After All, scholar Marjorie Garber writes that she's "brilliant, complicated, [and] strong" and George Bernard Shaw famously said that the countess of Roussillon is "the most beautiful old woman's part ever written." (Source).
We see a tender side of the countess when she overhears Helen talking about her love for Bertram:
Even so it was with me when I was young.
If ever we are nature's, these are ours. This thorn
Doth to our rose of youth rightly belong. (1.3.130-132)
The countess may be an old woman, but she remembers what it was like to be young and in love. She also remembers just how painful it can be. This is pretty remarkable, Shmoopers. It's not every day that you see one of Shakespeare's parents siding with the younger generation. (In Romeo and Juliet, for example, the young lovers' parents are a major obstacle to their kids' happiness.)
The countess's compassion may be why she is always building up Helen's self-esteem and trying to help her get Bertram. This actually has a big impact on the way the audience experiences the play. Because the countess is always telling us how great Helen is, we're encouraged to identify and sympathize with Helen, even if we might question the girl's actions or motives.