Rosemary's most notable characteristic is almost total fearlessness. She is remarkably impulsive. Although capable of frightening levels of violence, she does not seem to enjoy hurting other people. She is a very skilled combatant, quite intelligent, a quick learner and a skilled juggler. Although at first she was overwhelmed by the strangeness of every aspect of life in the Mansion, she has come to terms with the situation with remarkable quickness.
WARNING! MINOR SPOILERS!
Rosemary's father was Connor Sandhall Dapple and her mother was Paprika Pilkington (mother) in Moonin island. They died at sea while Rosemary was a very small child. They were heading to Gnomin to Genny when a storm sunk their ship. Rosemary was under quarantine with jibjib pox. She barely has any memory of them. She was brought up by Aunt Eva and her ex-bodyguard/friend Zeke.
Once she grew up, she decided to leave her home-island. She spent a great deal of time travelling and taking up various jobs. She was a farmhand from time to time, she "sharpened pencils" (likely means that she was an assistant) for a man named Erwing. She was even on rigging duty on a ship named Oshinoxtra. She eventually found gainful employment within Hack N' Slash mercenary company, where also she recieved training and some education.
There, she became a caravan guard disguised as a juggler. Her superior was Baldy. What happened to cause her to leave them is unknown. (Revealed in comic in 2016/2017)