The Kin numbered only one, but could time travel, and populated places by moving through time so there were many versions of it. The Kin needed other entities to ask it the time and create quantum superpositioning that allowed it to be anchored to a place-time location. The Time Lords didn’t kill any of the Kin as due to the Kin being one being at different points in time killing any would be genocide. They imprisoned the Kin in a prison out of temporal phase with the rest of the Universe. The First Doctor heard the story of the Kin while a small boy on Gallifrey, before he went to the academy. The true face of the Kin was thought of by Amy Pond as a “crawling, squirming, wriggling mess”.
After the Last Great Time War, the Kin was able to escape its prison. Wanting to lure the last of the Time Lords, the Kin travelled to Earth 1984 and began to legally buy the planet using money recycled through time. The Kin wore a variety of masks while on Earth to cover up its face, including a rabbit mask, a cat mask, Margaret Thatcher, and Amy Pond. In 2010, it had created an alternate timeline where humanity was extinct and Earth was legally its property.
The Eleventh Doctor and Amy found the Kin in control of Earth and travelled to 1984, where the Kin had recently killed a young girl called Polly whose house it had bought. The Kin disguised itself as Amy so it could access the TARDIS, and told The Doctor to go to the dawn of time to escape the Kin. From the beginning, the Kin planned to gain control of all time. However, The Doctor realised who the Kin was and travelled to a point before the Big Bang. The Doctor told the Kin he would have helped it, however because it killed Polly, he now refused to. When the Kin stepped out of the TARDIS, it was trapped before time forever and its actions were negated. (Nothing O’Clock)