What sort of character is circe? why (aside from her magic) is she able to turn odysseus' men into swine? what have they done to offend her, or what weakness do they show?

Circe is a goddess and also a sorceress capable of doing supernatural things just like the gods.

She was able to turn them into swine/pigs as she was also a goddess who can do anything she wants it done.

Odysseus' men have acted foolishly, greedily devouring the food, thus making their gluttony get the better of them. This led to Circe transforming them into pigs.

Explanation:

Homer's epic "The Odyssey" tells the journey taken by Odysseus and his men during their return to Ithaca after being away for more than two decades. It also reveals the various trials and temptations faced by the men on their way.

The men encountered Circe in Book X when they had been turned away from the other places and they were looking for a place to rest and get food. The island of Aeaean where they had arrived was owned by the goddess Circe, who, aside from being a goddess was also a sorceress. She was capable of making the men turn into pigs, locking them in a pigsty, and also making them forget all about their plans to return home.

The men were turned into swine as they were too gluttonous, not caring about their actions and having n manners except to get themselves full. This led to Circe turning them into animals.