Hugo doesn’t really know what he’s getting himself into when he meets Isabelle:
It was the girl from the toy booth. Hugo was about to yell to her, but she put a finger to her lips and motioned for him to wait there. The curtains closed again. (1.4.6)
She certainly does a good job of making a mysterious entrance—and a cinematic one, at that, with all those curtains closing. And she's a mystery in more ways than one. Isabelle doesn’t seem to be on Hugo’s side or Georges Méliès'. She agrees to help Hugo find his notebook because she thinks that it’s the right thing to do (or because she’s just curious and likes an adventure) but she also defends Méliès and loves him because he’s her godfather.
Though she may look like a sweet little girl with that Madeline haircut, Isabelle’s got as much spunk and street smarts going for her as Hugo does, if not more. After all, this is a girl who sneaks into movie theaters and steals a precious necklace from her godmother just because it’s pretty. She knows how to get what she wants. So when Hugo steals that very same necklace, she doesn’t take it sitting down:
Before he could cover the mechanical man, the door burst open. Hugo didn’t have time to scream as a dark figure lunged at him, knocked him to the ground, and landed on top of him. His head banged painfully against the floorboards. (1.12.5)
She may be a young girl, but Isabelle certainly isn’t the kind of prim and proper gal who’s afraid to get dirty and tackle someone who’s wronged her.
The best way to describe Isabelle is that she’s kind of like the Robin to Hugo’s Batman, except she’s a lot more of a handful. She wants to help Hugo, and she wants to go on adventures with him, but she’s not going to let him make all the rules:
“You better not look inside.”
“If I find it I should at least be able to look at it.” (1.8.22)
Basically, she tells Hugo like it is: if you want me to get your notebook for you, you had better expect I'm gonna take a peek. She's a curious girl, after all, and she wants to be a part of Hugo's wild adventures.
Isabelle may insert herself into Hugo’s shenanigans, but she makes them her own as well. If she finds that notebook, then she’s going to look in it, and if she goes to the movie theater and finds that Etienne can’t sneak them in, then by golly, she’ll get them in herself. She's a go-getter, and like Hugo, she helps set things in motion.
Though she’s more of a sidekick at the beginning (one that Hugo didn’t ask for), by the end she’s a true friend to Hugo; they're equals. Maybe it’s because they’ve been through all of these adventures and discovered all kinds of secrets together, or maybe it’s because they're both orphans, or maybe it's because they just understand each other:
Hugo had never told anyone the whole story. It had been his secret for so long that he wasn’t sure he even had the words. But he looked at Isabelle, and it was as if he could feel all the cogs and wheels begin to engage in his mind, and the words suddenly came together. (2.5.30)
Hugo’s never felt like he could share his story, but if anyone can understand what he's going through, it's Isabelle. She's an orphan, too, and just trying to carve out her own life for herself. She's got a makeshift family, sure, but she could use someone her age to hang around with. We can't imagine Papa Georges and Mama Jeanne are much of a party.
Perhaps that's what makes Isabelle such a good match for Hugo. She’s just as brave and adventurous as he is. She wants to go where the excitement is, and Hugo is all kinds of excitement.
The best part is, though, that they both respect each other (despite the occasional thieving). Hugo shares with Isabelle his life story, which he hasn’t ever told to anyone else. And he soon discovers her story, too. That brings them closer together, until she's much more than just a sidekick. She's a friend.