Walder Frey believes that the person who dies with the most sons wins. That's the only possible reason we can think of for why he has so many children, both from his eight wives and all the mistresses he's had over the years. Frey is an important nobleman who shows us that some nobles are looking out for themselves above all. (If you didn't already get that lesson from Petyr Baelish and the Lannisters, Walder Frey really drives the point home.)
Walder is an old and bitter man (ninety years old! ninety years bitter!), who is very jealous of other nobles. Apparently, the Frey family rose to power (and wealth) by owning this one bridge over a river and two castles (the Twins) to protect that bridge. So the other houses look down on the Freys as not so noble, and Frey just can't stand this. Seriously. He will go off at the drop of a hat about how the other nobles aren't any better than he is:
I'll wager you, [Lord Tywin] eats too many beans, he breaks wind just like me, but you'll never hear him admit it, oh, no. What's he got to be so puffed up about anyway? Only two sons, and one of them's a twisted little monster. I'll match him son for son, and I'll still have nineteen and a half left when all of his are dead! (60 Catelyn 9.91)
So Walder Frey feels like he's in a competition with the other nobles. He's hard to deal with, because he's so easy to offend, but at least people always know what he wants. That is, even though the Freys are sworn to the Tullys, Walder Frey always wants more respect from the old houses, and will withhold his help unless he gets it. So that's how Catelyn gets his help: she promises him that the Starks will marry some of the Freys. (Marriage = respect.)
Can We Trust Him?
Um, no. We can't.
Since Frey is bribe-able, there's always a danger that the other side will offer him more. Also, he only wants to be on the winning side. That's how Frey dealt with Robert's rebellion: he joined after Robert had more or less won. And so, Hoster Tully gave the elderly Frey the mocking nickname "the Late Lord Frey."
So Walder is the very picture of the scheming nobleman – and everyone pretty much knows it.
Minor Characters Connected to Walder
If you thought the Stark family was big and confusing, wait until you get a load of Walder Frey's family. Walder has had eight wives – the newest is a sixteen-year-old named Joyeuse (60 Catelyn 9.96). And he has around twenty-one sons, including illegitimate sons. Here are some of the Freys we hear about (just imagine how many don't show up in this book):
Stevron Frey, the oldest, who joins Robb Stark's war council
Jared, fourth son
Hosteen, sixth son
Danwell, eighth son
Merrett, ninth son, and father of Little Walder (who will go foster at Winterfell)
Jammos, thirteenth son, father of Big Walder (who will go foster at Winterfell)
Whalen, fourteenth son
Perwyn, fifteenth son
Olyvar, eighteenth son, who will serve as Robb Stark's squire
We also hear about Emmon and Theo Frey, and the illegitimate Martyn Rivers, all of whom take part in the Hand's tournament. And when Catelyn comes to negotiate with Walder, she also sees Walder's illegitimate sons Ryger Rivers and Ronel Rivers. This is one way that we know that Lord Frey doesn't really have too much social grace: he keeps his illegitimate children around. (Whereas the civilized thing to do would be to send them away and keep them out of sight. Well, this is one area where we kind of agree with Frey.)