Tony the Left-Handed
"I'm a left-handed guy," Tony tells Alice. He doesn't mean he's literally left-handed; what he means is that he's a bit of an odd duck.
Of course, Alice's family is filled with odd ducks; they're all bohemian goof-balls bouncing around the house dancing, painting and setting off fireworks—because setting off fireworks seems like a fun thing to do.
Alice's family, though, are all goofy in one particular way each; Essie dances all over the place; Alice's dad makes fireworks. Tony is a bit different. He's not eccentric, and he doesn't have any hobby that he's devoted to. Instead, he's just funny and quick-witted:
ALICE SYCAMORE: Have you ever been in a monastery?
TONY KIRBY: No, but I'm the fella who got caught in a cave once.
ALICE SYCAMORE: Were you? Whatever happened to you?
TONY KIRBY: Well, the cave caved in and I haven't been heard from since.
Part of this is Tony being in love and filled with joy and nuttiness (life is chasing around inside him like a squirrel, as Kolenkhov says.) But part of it is just Tony—and perhaps Jimmy Stewart, who was an incredibly charming actor.
In fact, even when Tony/ Jimmy Stewart ends up trailing off and bungling a joke, it's charming:
TONY: We can't just pop out of the building with no place to go. Very bad idea that, you know. I know two people that did that once—they went out of the building, they were uncertain, so they just walked and walked and walked and finally they just died... of hunger. Now you wouldn't want anything like that to happen... because if, if that happened... you're so beautiful.
For some actors, this dialogue would be cutesy. If Tom Cruise said that, Alice Sycamore would throw up in her mouth and run for the hills (not necessarily in that order.) But Jimmy Stewart can pull it off, because he is the Jimmy Stewart-est.
Tony the Sycamore
One goofy, whimsical thing Tony does is to deliberately get the date wrong when his parents are supposed to visit the Sycamores. This works out badly, and Alice is mad at him—but it's interesting that Tony thought this was a good idea in the first place.
TONY: I want them to meet you as you are, not some trumped up evening with everybody acting unnaturally.
The truth is that Tony seems almost as much in love with Alice's family as with Alice herself. He wants his parents to meet Grandpa & Co because he's proud of them. He thinks they're funny and fun and wise, and he looks absolutely delighted when Grandpa tells off the tax collector.
TONY [to Alice]: You know everybody's afraid to live.
But the Sycamores don't seem afraid to live from his point of view. Tony, stuck in his boring bank job because all Kirbys are bankers, finds that appealing.
When Alice breaks up with him at the end, Tony finally gets the courage to leave his job, and go pursue his dream of developing solar energy. Alice leaving makes Tony embrace his own inner dreamer. And after Alice comes back, it's not so hard to imagine that Tony might want to just stay in that old house with her, occasionally playing the xylophone with Ed.
Tony marries Alice because he was a Sycamore all along.