Jamie Kinkaid sure is a funny kid. As Claudia describes him, he's a total tightwad with his money, but that's why she takes him along. He's also obviously the most bearable of her siblings, and the two get along pretty well on their adventures. While Claudia is emotional and kind of snobby, Jamie is often the calming force.
Basically, they're the perfect team.
Think about it. When Claudia is sobbing her eyes out dramatically at the train station (how very French cinema of her, right?), he tries to go above and beyond for a little brother:
"C'mon," he pleaded, "we'll enjoy telling them about how we lived in the museum. The violin case can be evidence. Do you realize that we've lived there a whole week?" (8.53)
This kid is remarkably mature. Instead of going the usual sibling route and making fun of her until she cries harder, he supports her when she's going through a tough time.
There's no question about it: Jamie is hilarious. Claudia may not always agree (it depends on her mood), but we're amused for sure.
Jamie is quick on his feet—and even quicker when it comes to witty sayings. He uses this to his advantage quite a few times. When he mistimes his exit in the bathroom and a janitor catches him and asks him where he came from, he happily responds with, "Mother always says that I came from Heaven" (4.9).
Not only is this super gutsy, it's just flat out funny. Obviously the janitor wants to know why he's in the museum, but Jamie pretends that it's a question concerning, you know, the origins of man. It definitely gets him out of that sticky situation. And it definitely gives us a laugh.
Jamie's humor comes in handy at other times, too. Because he's more lighthearted than Claudia, he's able to cheer her up, or at least not get bogged down with her when she gets sad or frustrated. After they receive the letter from the museum officials saying that their secret wasn't really a secret, he's there to comfort his sister:
Jamie let her cry for a while. He sat there and fidgeted and counted the number of benches. She still cried; he counted the number of people on the benches […]
After the big blobs of tears stopped, he said, "At least they treated us like grown-ups. That letter is full of big words and all." (8.41-42)
He doesn't make fun of her, and he doesn't get frustrated with her. Instead, Jamie seems to know exactly how to deal with his emotional big sister. He lets her cry it out and then tries to make her feel better by saying that it's not as if the museum people totally wrote them off—they actually treated them like adults.
At times, Jamie may seem like he's just the silly younger brother. But there's a reason why Claudia didn't run away without him: she needs him there to keep things calm and lighthearted. Left to her own devices, Claudia might be way too serious and take things harder than she should. She lacks a lot of that fun, childhood spirit that Jamie brings to the game.
Jamie may be the younger brother, but he definitely had his own edge. Whereas Claudia just wants to spend money (she has expensive taste, after all), Jamie has a knack for keeping track of and raking in the dough. He'd probably fit right in on Wall Street. But instead he ends up with Claudia at the Met, where she appoints him treasurer of their little venture.
And he really takes the task to heart:
James was dismayed at the prices. They had $28.61 when they went into the cafeteria, and only $27.11 when they came out still feeling hungry. "Claudia," he demanded, "did you know food would cost so much? Now, aren't you glad that we didn't take a bus?" (2.27)
Claudia's a rash spender, but Jamie knows how to pull in the reins when she gets out of hand. And they need it—they only have a certain amount of money to spend during their adventure, a fact that Claudia doesn't seem to understand. Despite her straight-A's and advantage in age, Claudia needs Jamie around because he's the more financially savvy one.
In the end, they're both vital to the mission, which makes them the perfect sibling duo. They're practically superheroes together.