Thor lives up to his reputation in the myth of Thor and the Jotun Geirrod. If you want to know what his reputation is, check out Shmoop's files on him.
He's a fighter, not a thinker. Loki basically says, "Hey Thor, this giant's house is pretty awesome. Why don't you visit it without your weapons and armor?" And Thor goes, "Okay!" Now does that seem smart to you? Especially since Thor's been tricked by giants many, many times before? But luckily, being a bit dim is no problem when you've got colossal strength. Even without his hammer, Thor's a force to be reckoned with: The fatal crushing of Geirrod's daughters, and the general slaughter that follows, proves that.
He's decisive and quick to act. Thor's words to Loki when he realizes that Gjálp's big pee is the cause of the waves that are swamping them are "You've got to stem [dam up] a river at its source." That pretty much sums up Thor's approach to problem-solving: See problem. Find source of problem. Throw rock (or hammer) at problem. Problem solved.
He really, really hates giants. And you can understand why, given how Geirrod treats him in this story, first entertaining him in the goat house, then throwing a flaming iron at him. But even though it's really only Geirrod and his daughters that treat Thor badly, by the end of the story, he's killed all the giants in sight. That's just how he rolls.
Even though this story is mostly about Thor, it's Loki that gets it all rolling when his bird disguise fails, landing him squarely in Geirrod's clutches. What was Loki doing dressed up as a bird, anyway? Well, he's Loki. He likes to wear disguises, probably because he loves the idea of tricking people. Plus, he wanted to spy on Geirrod, probably because he enjoys knowing things he's not supposed to and being places he's not supposed to be. And besides, haven't you ever just wished you could be a bird?
But Loki's curiosity lands him in big trouble when Geirrod catches him. He's faced with a moral dilemma: Does he lure Thor into Geirrod's territory weaponless or remain locked in the box forever? Since Loki's not really loyal to anybody but himself, it's no surprise that he agrees to hand Thor over on a silver platter to save his own neck.
What is surprising is that he actually keeps his promise once he's back in Asgard. Why is anybody's guess, but here are a few of ours: (1) Oaths are really, really important in Germanic culture. Even Loki won't break one. (2) He's actually kind of scared of what Geirrod might do to him if he doesn't keep his promise.
We (and a lot of the Asgard gods) tend to think of giants as stupid, but they actually come in all shapes and (brain) sizes. Geirrod's brain is one of the bigger ones. He's not at all fooled by Loki's bird disguise, and he's pretty quick to figure out how he can use his catch to get the person he's really after: Thor. Even his plan to kill Thor by enticing him to play a "game" of "catch the molten iron" is kind of clever, since it takes advantage of Thor's natural competitiveness.
Nah, Geirrod's no dummy. As Grid describes him, he's a tricky son-of-a-gun. But he also has a cold, cold heart. When his daughters are gruesomely crushed under Thor's chair, he doesn't even bat an eye. That makes it hard to feel sorry for him when Thor returns his molten iron right through his stomach.
Every quest needs the wise old mentor who guides the hero on his way. In this story, that role is played by Grid. Grid's a giantess, which tells us that not all giants are cut from the same cloth. Plus, that puts her in-the-know about who's who in the giant world: She knows that Geirrod is not to be trusted, and she's happy to share that information with Thor. Why? Well, she might be helpful out of the goodness of her heart. But she's also got family ties to the thunder god. She's Odin's mistress and the mother of Thor's stepbrothers, so her eagerness to help Thor could come from an abundance of family feeling.
That family feeling is something Grid has that her foil, or contrasting character, Geirrod, doesn't. Grid's presence makes this lack in Geirrod clear by contrast. Grid's great hospitality to Thor also contrasts with the way Geirrod treats him.
We've said it once and we'll say it again: Grid's the yin to Geirrod's yang.
Geirrod's daughters are a bit, um, weird, to say the least.
The first one Thor meets, Grápr, decides to straddle the River Vímur and pee. And since she's a giantess, her urine flow is no small thing. But Grápr's probably not just peeing because she just drank six venti Frappuccinos. Nope, she's probably just loyal to dad and wants to help him best an enemy.
All of Geirrod's daughters demonstrate allegiance to Geirrod in this way, the other two attempting to crush Thor against the ceiling by giving him a chairlift ride on their backs. Geirrod certainly doesn't deserve such loyalty, but by standing with him against Thor, these girls prove themselves to be good Viking daughters.