First of all, the idea sounds great. Ok, we see many tomboys but I really like something about her as a whole. It's like a combination about two of my characters, one of them a hunter girl that was practically raised as a boy, and another a pretty girl who was almost married off to a man a couple of decades her elder, so she fled home. Ok now those sound a bit boring when I represent them so quickly... *sigh* And hey, not to mention my drow character who was the younger daughter, spend most of the time locked in her room but levitated out of the window to the yard and started playing with slave boys. Something familiar?
She ended up being a mage since they had no other candidates and the house had to be protected from supposed threats. Though then she screwed it up and went rogue.
Anyway, why do you want to get Rea interested in magic? Arranging an incident in her past that includes some sort of magic, there are many options for that, but is magic really needed? I have sometimes noticed it's a real pain in the butt...
A quick idea for a story. She decided to run away for a little adventure with the boys, they started playing hide and seek and she stumbled across a badger's nest. At least she figured it was badger's since she didn't know much about wild animals other than those dusty rugs in his father's library, and he had always scowled at her for petting them and told her they were no toys.
"Eight... niner.... tenner! Ready or not, here I come!" screamed the seeker with boyish glee.
Rea ducked quickly. Oh no, he was heading her way... The black hole gaped in front of her, inviting her into the dark safety. Taking a deep breath she started crawling inside. The air smelled of soil and something bitter, badger pee perhaps?
The hole soon extended into a cramped cave. Soon feelings of clausthrophobia filled her and she started turning around to at least be able to see outside. ("Hah, I caught Tommy!" the seeker hollered outside.) Rea's eyes set on something gleaming next to the tunnel wall, reminding her of the glass eyes of the poor bearskin in front of the fireplace. Wow, a new marble! she thought. It was waiting for me all along, I bet I win all the games with this! She pinched it and pulled but somehow it seemed to be stuck though that didn't make any sense at all, the wall was just earth, it should have come loose... She started digging around it, completely forgetting her fears. Yes, the marble, or was it even a jewel of some sort? seemed to be attached to something, a small metallic hilt, and when there is a hilt, there is also a blade... A final tug and the dagger was free.
Now she might release a power, or probably start digging for more, finding the sheath and then a skeleton of the old carrier, but at that time the cave will start collapsing. Her friends will hear her screams and find her half buried alive and unconscious. They quickly dig her up and she is sure to have a trauma for life... probably preventing her from remembering the incident until years later. Dream sequenses trouble her though, and she gets an odd feeling when observing kids play with marbles.
Ok, maybe my imagination is running too wild today...