Lake, also known as the Great Lake, was a clone of River Song and the leader of a religious cult.
Lake was created on Demons Run by Madame Kovarian, along with at least six other clones. Sometime after escaping Demons Run, he was tending to fuel cells with Wadi and when one of the cells blew up in his face and the explosion caused 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 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 it’s he focused on Quisling’s face, he could take on that form for his next body, he did this and began working with Sharon, thus taking Quisling’s place.
Later he took Lily captive and killed her in front of River Song, realising that she was a proto-Timelord but not knowing where she came from or who she was. Lily didn’t regenerate as she had usernames 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 earljee incarnation killing her for the cycle to be complete. (AUDIO: The Lady in the Lake)