Fly is an incredibly sweet big brother to Stella. Not only do they not have the type of siblingdynamic usually depicted in such stories, Fly is repeatedly shown to be very protective of Stella which, given the two siblings' very apparent age gap, makes a lot of sense.
It apparently never crosses Fly's mind to just leave Stella at home with Aunt Anna instead of letting her tag along for the fishing trip.
When Stella, having unwittingly downed some fish potion while Fly had his back turned, winds up lost in the ocean, Fly takes the potion himself and jumps headfirst into the sea to go look for her. When he does find Stella, he never once berates her for what happened in the lab and instead points out the fun sides of being turned into fish.
In fact, Fly frequently tries to distract Stella from how scary the situation actually is, namely turning their hunt for the ingredients to the antidote into a fun game.
Even when he's badly hurt at the climax, Fly still tries to downplay his injuries so as not to frighten Stella - not that it does much good.
Chuck and Fly begin the story just about tolerating each other; by the end of the film, their dynamic has shifted to newfound respect and genuine friendship.
Chuck isn't best pleased at having been turned into a jellyfish, especially since Fly on his part thinks this is hilarious, but Stella tries to make him feel better about it by telling him he's a cute jellyfish. Aww...
The fact that Bill and Lisa take enough of an interest in their kids that they can piece together where they might have gone, just from Fly's missing lures and fishing rod.
