Sara is the only survivor of the shipwreck besides you.
In the introductory video the player (the Nameless Hero) and Sara are stowaways on the Inquisition's boat during a storm, during the event scene the boat is destroyed by paoki trelo massive tidal wave and the Nameless Hero and Sara are washed away to the shore of Faranga island.
At the beginning of the game, both Sara and you wake up on the on the beach, just a few steps apart. When you talk to her, Sara asks you to find a way out of the place. She also mentions that she saw "something" moving in the bushes — so you better equip yourself with some sort of a weapon. Once you have done that Sara will ask you to lead the way through the trail leading into the island. She will also ask you to find something to eat to help get back to strength. You will find a couple of hungry Sea vultures around the beach. Kill them and take their meat.
Down the trail you will come across an abandoned house with a fire at the front. Roast the vulture meat in the fire using a Frying pan and share some with Sara. She will decide to take a break there and ask you to go ahead.
Further up on the trail, you will meet Jan. If you tell him about Sara, he will rescue her, and send her to the bandit camp. This will earn you the Philanthropist achievement. You can also omit telling about Sara to Jan and on the way to Harbor town tell Tristam about Sara instead.
Other than that she remains an unessential character throughout the story.
Washed onto the beach, relaxing in front of a hut (near beach).