Rindle was a Proto-Time Lord clone of River Song.
Rindle was created on Demons Run by Madame Kovarian, along with six other clones. After the Eleventh Doctor raided Demons Run, Rindle escaped on a ship with the other six clones. Afterwards, they arrived on a planet where Rindle was subsequently killed by a wild beast. He was the first to discover the ability to regenerate, and upon discovering him in his new form, Lake murdered him, believing he was a shapeshifter posing as Rindle. Later Rindle regenerated again, this time into the form of a woman. She made her home in the Trees, fearing to return to the ship. She was later discovered once more by Lake, who killed her once again to power the ship. She regenerated into an older man before being killed once more by Lake.
Eventually, all of Rindle's regenerations were used up and they died permanently, with Lake using their regeneration energy to power the ship. (AUDIO: The Lady in the Lake)
Behind the scenes
Like all of her siblings, Rindle's name is a reference to water, with her name coming from the word rindle, which is a small watercourse or gutter.