H-One

H-One was a Proto-Time Lord clone of River Song.

She was part of a second batch of clones created by Madame Kovarian after River failed to kill the Eleventh Doctor at Lake Silencio. Her twin clone was H-Two. Like River and her fellow clones, H-One was raised on bedtime stories about the Furies and trained to be an assassin. She grew up considering Kovarian to be her mother and believed Kovarian's promise to give her a proper name after her first mission. However, H-One was shot and killed by Brooke during an argument and both her hearts were stopped so she could not regenerate. Brooke then tried to blame her death on River. (AUDIO: The Furies)

Behind the scenes
Like all of her siblings, H-One's name is a reference to water, along with her sisters H-Two and O, her name is a pun on H20, the chemical formula for water, a water molecule is composed of two hydrogen atoms (H-One and H-Two) and one oxygen atom. (O)