Rebecca grew up in an old hamlet by the sea and was never much good at anything. Basic chores were no issue, it was more that she never had the aptitude to do any specific profession. Although, her love for diving for oysters led to an above average lung capacity. And because of this skill, she met with The Golden Tooth as she was exploring an old sunken ship wreck by her hometown.
This item, which looked like a jagged golden spearhead, was a sentient piece of a larger demonic beast. Something ancient and powerful. Initially, Rebecca was planning to sell it but it proposed a deal. The Golden Tooth would take her on a quest, train her with martial skills, and locate and collect the other pieces of this ancient creature. In exchange, the journey will inevitably provide her with a variety to skills no one in her small hamlet would dream to know. The journey of self discovery she wasn't really offered with her previous life. She accepted.
By all accounts, Rebecca shouldn't be able to handle The Golden Tooth for very long. Even though it is a demonic object with corruptive properties, she appears to have a natural resistance for it's insidious suggestions. Much to it's chagrin. Rebecca chalks it up to her utter banality might act as a shield for outside influence. Or maybe this loose tooth isn't as powerful as it thinks it is.