Stereotypical Boring Banker
When you think "wealthy banker" you probably tend to think, "boring busy middle-aged dude." And Kirby is pretty much that boring busy middle-aged dude. He's basically the Monopoly Man. He wears suits and orders people around and answers phones and mutters about money. He's totally pleased with himself, and occasionally boasts about how tough he is. He's like a super villain, but without the colorful costumes or the powers to make it entertaining.
Kirby's a snob too, as you would expect from a stereotypical banker. He is pleased with his own importance, and sneers at folks further down the social ladder, like Alice, who he thinks isn't good enough to marry his son. He calls the folks in prison "scum," which really riles up Grandpa, who says:
GRANDPA: When your time comes, I doubt if a single tear will be shed over you. The world will probably cry, "Good riddance." That's a nice prospect, Mr. Kirby. I hope you'll enjoy it. I hope you'll get some comfort out of all this coin you've been sweating over then!
It's harsh, but there's something to it. Who cares about Mr. Kirby? Who can even tell him from all the other jerky bankers? He's just the boring foil for all the fun characters in Grandpa's house. If Frank Capra had decided to use a cardboard Mr. Kirby instead of actor Edward Arnold, who would notice the difference?
The Carefree Eccentric, Mr. Kirby
But Mr. Kirby is deceptive. He may seem like a big old boring portly banker, but he has depths. Over the course of the film you learn he's had hobbies in the past: he likes to play the harmonica, and he used to be a wrestler. He's also very committed to his son. He makes Tony a vice-president, and then wants to make him president. He even laughs at Tony's jokes… which can be cornier than Iowa in July.
In fact, it's his love for his bouncing baby boy that makes ol' Kirby crawl out of his Monopoly Man shell. Once he realizes that his son is angry with him, he starts to come around to Grandpa's wisdom. Just check out this snippet of dialogue:
MR. KIRBY: Mr. Vanderhof, you once told me I was a failure as a father.
GRANDPA VANDERHOF: Oh, I didn't mean that.
MR. KIRBY: I know, but I am.
Aww. Isn't that sweet? We realize, in fact, that a lot of Kirby's money-grubbing has stemmed from looking out for his son in the way that he thinks he should. He makes a lot of money… for his son. He dislikes the Kirbys because a) they won't sell him their house, but also because b) he doesn't think they're good enough for his precious Tony:
MR. KIRBY: Boys like Tony don't marry stenographers.
But once he realizes that Tony has found true love with one very special stenographer, old Kirby realizes that he wants to be more like Grandpa. Grandpa teaches Mr. Kirby not to fear, and so Mr. Kirby starts embrace his inner harmonica player. In fact, now that his stuffy notions of duty to his son have gone out the window, Kirby can afford to do some soul-searching.
Of course, in real life, bankers don't usually sell back your house because you've leant them a harmonica. But perhaps Mr. Kirby was always a nice banker all along (even if he did hound poor Ramsey to his death). Or maybe Hollywood is nicer than real life. In Capra's world, even bankers are good eggs.