The only way we know anything about the banker or the lawyer is through the scanty descriptions we get from the narrator. Everything we know about the banker, we know from a few lines the narrator throws in when the bet is made: "The banker, spoilt and frivolous, with millions beyond his reckoning, was delighted at the bet. At supper he made fun of the young man" (1.11). And there you have it—he's rich, he's free with his money, and he's pretty cavalier about the lives and feelings of other people. A real winner.
He pretty much stays that way throughout the text, unless you think that his tears at the very end of the story mean that he's had some kind of dramatic transformation. (What does Shmoop think? Check out the "What's Up With the Ending?" section for our two cents.)
The lawyer, on the other hand, doesn't get a solid description until the very end of the story, after he's been locked up for fifteen years without any human interaction:
At the table a man unlike ordinary people was sitting motionless. He was a skeleton with the skin drawn tight over his bones, with long curls like a woman's and a shaggy beard. His face was yellow with an earthy tint in it, his cheeks were hollow, his back long and narrow, and the hand on which his shaggy head was propped was so thin and delicate that it was dreadful to look at it. His hair was already streaked with silver, and seeing his emaciated, aged-looking face, no one would have believed that he was only forty. (2.10)
Still, instead of any kind of personality or character description, what we get instead is a picture of a really strange, kind of alien man. What do we make of the husk he's become? Well, the fact that he no longer looks human sets up really well the revelation in his letter—that he's giving up the human material world in favor of a purely spiritual life.
The two men's actions are just strange enough to say something about them. But what? That, Shmoopers, is hard to say. Still, let's check out the odd things they do anyway.
First, we have the banker. This is a guy who turns an abstract discussion about freedom into a pretty grotesque and very real bet and then agrees to turn his guesthouse into a prison cell for fifteen (fifteen!) years. Are these the actions of someone whose wealth has spoiled him? Or does this set up nicely the idea that the banker is totally callous about human life—exactly the kind of guy who would then decide to simply kill the prisoner instead of dealing with having to pay up his side of the bargain? Maybe it's a little bit of both.
And then, the lawyer. Now this guy's a dude who ups his own side of the bet from five to fifteen years imprisonment, which shows that he is both a bad poker player, and probably at least a little bit bonkers. He doesn't even demand more money for this raise of his own ante, for crying out loud. Here's some advice, good sir—stay away from Vegas, because you really don't get how the whole betting and raising thing works.
Still, this strange gesture does hint pretty well at what will happen to the lawyer in captivity. Just as he was eager to throw away an extra ten years of his life for no good reason (in Shmoop's humble opinion), so he jettisons most of his humanity when the term of the bet is up.