From Portals of Phereon Wiki

Sharra can be found in water tile events, in tundra portals. Her event consists of her being injured and requiring aid. After healing a bit, she wishes to continue fighting. To recruit her you must choose to help her fight, and win a battle. When recruited she comes with a spear, a helmet and some healing herbs. The combination of rare portal type and getting the right RNG on the right type of tile makes her one of the most annoying NPCs to recruit; you should definitely Stabilize any Tundra portals that show up if you want to unlock her.

Combat wise Sharra is stronger than a regular mermaid with higher base stats and better strength growth.