Rose learned 3 things quickly growing up in Alkenstar: Guns are the Law and the Art of the city. You couldnt go 10 feet without seeing them, and everyone and their cousin wanted the best. And that was the second thing she learned; Her father, and their shop, Thorny Sides, was one of the best around in their own specialized niche. They dealt with all variety of small arms, concealed weapons, and self-defense implements. If it could be hidden, incorporated into wearables, or palmed, they made the best. Tiny hands really helped in that regard.
But early on she learned the hardest lesson: Her father despised her. She was the product of a deal struck with a cunning hag and himself, and caused the eventual split between her parents. He had made the deal to sabotage some upstart competitors in the same market, and she was the price. He really had no choice but to teach her the family business, seeing as she was the only family left. He didn't make it easy, but she did learn everything he knew. And more when she decided to do her own tinkering and research.
After a particularly nasty falling out, he attempted to gouge out the eye that showed the proof of her hag heritage, scarring her deeply but failing. In retaliation, she took his hand, and fed it to the furnace. They went their separate ways after that, but not before she made her own weapon of choice: Scarlet, proof of who she was and where she came from.
Hearing of the portal leading to new, unexplored lands, she jumped at the chance, and brings her knowledge and expertise with firearms to the New World and First Landing.