Brooke (The Lady in the Lake)

Brooke was a Proto-Time Lord, grown from River Song's DNA, and a companion of the Fifth Doctor.

Creation
Madame Kovarian harvested Melody Pond's embryonic DNA in order to grow more children like her. This resulted in the creation of the Proto-Time Lords, the first batch consisted of seven siblings: Lake, Rindle, Tarn, Wadi, Creek, Stream, and Beck. (AUDIO: The Lady in the Lake)

The second set of siblings included Brooke, H-One, H-Two, and O. (AUDIO: My Dinner with Andrew, The Furies) Brooke and the other Proto-Time Lords had the ability to regenerate; however, the exact amount of regenerations they each had was random. (AUDIO: The Lady in the Lake)

Incarnations of Brooke
Brooke's first incarnation had been sent on a mission by Madame Kovarian to kill the Doctor and joined him on his travels. (AUDIO: The Furies)

After her previous incarnation shot the Fifth Doctor using regeneration inhibitors and left him dead without the ability to regenerate, River retaliated and shot Brooke, triggering her regeneration into a new incarnation and revealing her status as a Proto-Time Lord and one of River's siblings. (AUDIO: My Dinner with Andrew)

Behind the scenes
Like all of her siblings, Brooke's name is a reference to water, with her name coming from the word brook, which is a small stream.