H-Two

H-Two 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-One. Like River and her fellow clones, H-Two 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. She repeatedly tried to attack River after H-One's death because she believed River had killed H-One. Once she realised River wasn't responsible for H-One's death, she agreed to join forces with River against Kovarian. In an attempt to make H-Two reconsider, Kovarian claimed she was going to give her the name Nadi, but H-Two remained faithful to River. After Kovarian was defeated and the Doctor was saved, H-Two decided to stay with Brooke 2 and O so they could torture Kovarian until she either died or they could find a way around their neural blocks to personally kill her themselves. (AUDIO: The Furies)

Behind the scenes
Like all of her siblings, H-Two's name is a reference to water. Along with her sisters H-One 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).