Lake was created on Demons Run by Madame Kovarian, along with initially six other clones. Sometime after escaping Demons Run, he was tending to fuel cells with Wadi when one of the cells blew up in his face, the resulting explosion causing him to regenerate.
Lake 2 was taller than his previous incarnation, looked younger, had a thinner face, had different coloured eyes and hair, was left-handed, and had an allergy to cheese. At first, the other clones did not believe he was the same person, despite Wadi's confirmation of witnessing his transformation, but eventually they grew to believe him and regarded him as a spiritual leader of their group.
Lake grew obsessed with his own mortality and couldn't stand not knowing how long he could cheat death. After discovering that Rindle also had the ability to regenerate, Lake alienated him from the group and covertly watched him. After Rindle regenerated into a female incarnation, Lake found her, killed her, and continued killing each of Rindle's subsequent regenerations in order to study the process and find out how many lives Rindle had. When Rindle's lives were used up, Lake resolved to convince the remaining members of his group to seek continual rebirth so he could secretly continue studying their deaths.
Lake later met Mr Quisling in a bar and they began to talk about the Euthanasia program on Terminus Prime. Knowing that some of his cult followers would be there, he killed Quisling and was also forced to regenerate in the same encounter. Lake realised that if he focused on Quisling’s face, he could take on that form for his next body. He managed to do so, and began working with Sharon, thus taking Quisling’s place for three months.
Later he took Lily captive and killed her in front of River Song, realising that she was a proto-Time Lord but not knowing where she came from or who she was. However, Lily didn’t regenerate, as she had used up all her allotted regenerations. Furious, River shot Lake dead, only to be shocked when he regenerated into Lily. Realising what had to be done, River sent the newly regenerated Lake back in time so that River and Kevin could encounter her in the cave prior to her earlier incarnation killing her for the cycle to be complete. River then buried Lily's body. (AUDIO: The Lady in the Lake)
Behind the scenes
Like all of his siblings, Lake's name is a reference to water, with his name coming from the word brook, which is a large area of water surrounded by land.