Squeak was the nickname of Midge’s younger sister, Abigail. (At Childhood’s End) The two resided with their mother in a flat in Perivale. Squeak and Ace knew and liked each other.
In 1989, after Midge returned to Earth from the Cheetah World, infected with the Cheetah Virus and under The Master’s control, Squeak saw her mother being transported to the Cheetah world and her cat being killed by the “bad cat” The Master brought. The Seventh Doctor and Ace later arrived and found her crying. After learning what had happened, Ace took her to her grandmother’s flat upstairs. (Survival)
By 2020, she was going by the name Chantelle and working as a model. She was still a close friend of Ace. (At Childhood’s End)
Squeak eventually died in the early 21st century when her immune system gave out, having been weakened by severe pollution. (Cat’s Cradle: Warhead)