Gemma Griffin

Gemma Griffin was a Human, she lived in the town of Folkestone, she and her brother Samson met with the Eighth Doctor in a library and had several adventures together.

Travels with the Doctor
After meeting the Doctor in a library, Samson and Gemma followed him into his TARDIS. The doctor and the two siblings traveled to a number of planets, including Porteus and Murgatroyd. They also visited the Ice Caves of Shabadabadon, the court of Queen Elizabeth I, prehistoric Earth and Studio 54.

They also paid a visit to a friend of the Doctor, Edward Grainger, in 1956. (ST: The Long Midwinter)

Encounter with Davros
After a trip to Valuensis the trio came across a Nekkistani time cruiser, the two siblings, Samson and Gemma, went on board to explore, but there they encountered Davros, he took control of their minds, and forced Samson to knock out the Doctor, then Davros used his control of Samson to control the TARDIS, piloting it back to earth, Davros then altered the Doctors memory so as he forgot all about the two siblings.

The Doctor went on his way, and met Charley Pollard and C'rizz, Samson remained subconsciously aware of the Doctors travels, and Davros used this to monitor the Doctor. Davros had infected Gemma with a disease which mutates humans into Daleks, and he used this to pave the way for the Dalek invasion of Earth.

The Doctor, Charley and C'rizz arove on Earth and Davros sprung his trap, C'rizz joined a small resistance of Daleks, headed by Gemma, and they attempted to make his the new Dalek emperor, this attempt was unsuccessful and in the end the Doctor blackmailed the Daleks into leaving earth, which they did, taking davros with them and releasing C'rizz, who they had captured, and Samson, from their control. (BFA: Terror Firma)

Gemma's fate is unknown, although it is implied that she was killed by C'rizz.