Like, I imagine a freckled, fat-faced little critter with a "1700s Nursery Rhyme" bob haircut, the classic singular buck tooth, and little black dots for eyes. They would wear an apple, from which only their head, arms, and legs would stick out. The apple's top part is a hat. They'd have the classic, almost knee-length socks, and shiny black shoes that look slightly too tight.
In other words, a kind of homely fat kid in a Victorian-era school uniform, stuffed into an apple. Or a Wario World-ified version of that girl from WW:Gold.
During long falls, you would see black shorts and a wool vest peeking from the apple's bottom, almost like it's a costume for a play despite clearly being an actual apple. Fairy wings made out of shimmering pink light are connected to the apple, yet are clearly controlled by the kid. Birds pecking the apple (in specific in-game regions) would seem to cause pain. None of this would ever be addressed, either.
Would look like they'd belong in Wario World. Not as cartoony as Wario besides the face. The apple would be photorealistic. Little fat hands would have visible fingernails. The socks & uniform would be pretty realistic, but the dress shoes would actually look like SpongeBob's until you got close enough in Debug mode or whatever. "Though they appear solid black, the shoes are fully modeled, with multiple layers, yet oddly reflect the lighting very little in-game" kind of thing. Just because.
As for the story:
Wario gets "cursed" with having to take them along. After he gets over it, he sees them as a distraction at most, usually. The brat has a character arc as they interact with NPCs and experience the world. Wario is consistently unmoved, only listening to people as much as he has to. His thoughts on everything should be almost as cryptic as they were in 4 and World.
I think it'd be funny if, instead of the usual thing of acting like the silent protagonist is saying things between dialogue, the game would clarify that Wario is actually being silent, that his voice clips actually are the only time he makes noise. But it'd take a while to discover it, because most of the NPCs do the typical "the game pretends you said something back to me that I'm now reacting to" like you'd expect.
The apple kid (called Sugarplum or something), as they become better at listening or something, would have a small chance of randomly stopping their latest climbing section diatribe (usually about the scenery, weather, food, etc.) out of nowhere.
"You know, you never respond to ANYTHING anybody says. It's very rude!" (says the kid who barely ever let anyone speak, at first) "I always have to do it for you." (brief pause) "I just think, what would be the greedier choice, and answer like that!" And the player realizes that they were always just assuming the kid was adding on to their Yeses and Nos.
And, just when you think that's the punchline, just this once, Wario shakes his head and says "nuh-uh!" When the kid insists, even, he doesn't respond. So, they just sigh.
This would be a one-time occurrence, and it would be so rare that it'd take like 3 days post-launch before anyone started to post about it, and most people would have to look it up.
