Catelyn Stark is Mama Direwolf. She was born Catelyn Tully (rival family alert!) but became a Stark when she married Eddard. Her younger sister is Lysa Arryn and her younger brother is Edmure Tully. She's the mother of Robb, Sansa, Arya, Bran, and Rickon – and don't you forget that (being a mother is what she does best). Catelyn abducts Tyrion Lannister, whom she blames for the attack on her son Bran, and later helps advise Robb about his army.
Catelyn Stark is like sitcom mom from the 1950s, like Leave it to Beaver or Father Knows Best: she means well, but she sometimes messes up (like when she captures Tyrion and then lets him get away).
In this way, Mrs. Stark is kind of like a female version of her husband, Eddard. There's one big difference, though. Whereas Ned is a great dad even to other people's kids (like Theon Greyjoy), Catelyn is very focused on her own kids – and not even on all of them, for that matter. For instance, when Bran is in a coma, Catelyn pretty much forgets her other children momentarily (15 Catelyn 3). And even when Sansa and Arya (and Eddard) may be in danger, she thinks only about protecting her son Robb by sending him home to Winterfell (56 Catelyn 8).
This doesn't mean that she loves some of her children more, it just means… well, okay, maybe she loves some of her children more. But she's still a devoted mom. For an extreme example of this, check out how she even thinks of winning a victory over Eddard for her children's sake (7 Catelyn 2.67).
Our friend Cat is really good at being sneaky. But not bad sneaky: more like helpful sneaky. For instance, she is able to help Robb see the right choice about who should lead part of his army without coming right out and telling him (56 Catelyn 8). That's a clever way to get the right result without hurting her son's feelings. By doing this, she also helps Robb learn how to be a better lord. (See? A little sneakiness never hurt anyone. Well, at least not this time.)
Catelyn is also able to negotiate safe passage with Walder Frey (60 Catelyn 9), a very bitter old man. This is something her husband sure couldn't have done. (You know what else Eddard couldn't have done? Given birth to five kids. Doesn't Catelyn Stark deserve some rest after that?)
As much as we love Cat, we have to admit there are two things about her that bother us: (1) her hatred of Jon Snow; and (2) her abduction of Tyrion. The first is just something that we have to accept: while Cat may be a wonderful mother to her children, she just doesn't care about other people's children (even when we do). She doesn't so much mind that Eddard had other children (at least, that's what she tells herself), but she minds that he brought Jon Snow to grow up with her kids (7 Catelyn 2.89). The nicest way to put it? She's very family-oriented.
As for abducting Tyrion, we're not sure that was a smart move, even though she did it brilliantly (even Tyrion admits that [32 Tyrion 4.30]). She seems to jump to a conclusion about how the Lannisters were responsible for attacking her children and then starts off a major feud between the Starks and the Lannisters before anyone is ready for it. But still, we can at least see where this all came from: once again, love of her children. That is, if Tyrion was responsible for the assassination attempts on Bran, she's going to make sure that Tyrion never gets a chance to hurt her family again.
Bottom line: this lady loves her children.
Reality check: Catelyn was twelve years old when she was betrothed to Brandon Stark (7 Catelyn 2.16). Bring that up to your parents the next time they tell you you're too young to date. (We're not sure how old she is now.)
Let's keep in mind that Catelyn Stark was originally Catelyn Tully. This means she's not a native northerner and can give us an idea of what Eddard and his northern country look like from the outside. Pretty good deal for us as readers.