Do you like jesters? The whimsical entertainers of the royal court?
Stephanie was born to a family of royal entertainers and court jesters. Frankly, that was the life. Her family was loved by all: The king, the citizens, neighboring kingdoms, anyone who'd seen them perform. Their amusing antics entertained everybody, they were treated well, and you can fill in most of the large paragraph from Carl's cast page here. They were well-trained, too; it's hard work, really, and years of exercise and practice from a young age made Stephanie an ideal acrobatic performer, just like the rest of her family.
Unfortunately, that's not how Stephanie saw it. Where everyone else heard the joyous laughter of an entertained audience, she only heard the laughter. Where the rest of her family saw hard work and practice result in incredible, borderline inhuman acrobatic talent, she only saw hard work. And most importantly, where everyone else saw a beloved king who respected his entertainers, she only saw a king. Or, more exactly, a person responsible for all her hard work. And the laughter. The never-ending laughter.
What she ultimately wanted was power. And, the power she wanted was, appropriately, also ultimate. She's the sort that feels she deserves to command fear, respect, even right up to outright worship from people. She wants control, plain and simple. And most importantly, deep down, she wants to make sure nobody ever laughs at her again.
To this end, she felt her best bet would be to learn magic. After all, she's seen some of the people coming through the castle do amazing, powerful things with it, so that'd be a good place to start. It's just that she never quite got around the fact that it takes a lot of patience and discipline to do much with magic, and she wants that sort of power now. She's not all that good of a mage, is the point.
Of course, that's not saying she doesn't have other skills. After all, she has her incredible acrobatic talents, for one. She's also quite dangerously intelligent, if not impatient. She's just a somewhat lousy mage. Though, she knows how to make do with what little magic skill she has, and she CAN pull something big off if she sits down to it for long enough. After all, she did manage to mutate an ikki bird into a gigantic monstrosity of a pet, turn Simon into a humanoid frog, and murder her king in cold blood before leaving the castle. That's worth something.