Exact details of background will depend on how we're starting, but the basic gist is that she is from completely outside the area. Returning from a hunting trip, she found her family killed by bandits who had stolen what few valuables they had as rural peasants. Finding their tracks, Gwenill managed to track the group of bandits, and she followed them to their camp. When night fell, she had her revenge - she took them by surprise, and slaughtered them all.
When Gwenill rode into the local hamlet, she was almost unconscious, barely holding on to the saddle. After she was nursed back to health, Gwenill sold her family's farm, and with the profits she bought a full set of equipment for combat. Prepared to continue cleaning up the world and desperate to try to help innocent people avoid her family's fate, Gwenill set off away from her village.