Andrea

Andrea Yates was a school friend of Sarah Jane Smith who was used by the Trickster to remove Sarah from existence and thereby create an alternate reality.

Timeline 1

Andrea died on 13 July 1964 at the age of thirteen while she and Sarah Jane were visiting a pier. After a brief encounter with a bizarrely dressed girl who claimed to be from the future, at Andrea's insistence the two entered an out-of-bounds area. A railing gave way, causing Andrea to fall to her death.

Timeline 2

Just before Andrea fell, the Trickster — travelling through time to find a key moment in history that would allow the random destruction of Earth decades later — offered her a chance to switch places with Sarah Jane. The desperate girl agreed and a new timeline was created in which Sarah Jane died in Andrea's place. The Trickster then gave Andrea a Verron puzzlebox which made her forget about her deal with him, but she remembered what happened to Sarah Jane.

Andrea became an artist and eventually moved into 13 Bannerman Road across the street from Alan Jackson and his daughter, Maria. Indeed, no one else remembered her until Maria, who had been been protected from the Tricker's meddling thanks to a second Verron puzzlebox given to her by Sarah Jane, remembered Sarah Jane and challenged Andrea.

Andrea renewed her agreement with the Trickster, which resulted in Maria being taken out of time. This time Alan had the puzzle box. He remembered his daughter and retrieved her from Limbo. With meteorite K67 threatening to destroy Earth and Sarah Jane the only person able to save the world, Andrea repudiated her earlier agreement.

Timeline 3

With her deal with the Trickster taken back, Andrea apparently returned through time to the point when she originally died, with her and Sarah Jane's places switched back. She asked Sarah Jane to remember her, before falling to her death and restoring the original timeline.

Sarah Jane admitted that the death of Andrea was a catalyst in how she chose to live her life.

Biography from the TARDIS Data Core article, licensed under CC-BY-SA