Geraldine Diamond

Geraldine Diamond, or COLT-5000 when she was a cyborg, was originally a human art teacher from Mirablis 5.

In her original timeline, Geraldine was taken to be assessed for usefulness. Being deemed worthless to society, Geraldine was converted into a cyborg bounty hunter by Seavus Colt, later hired to find Jenny for her valuable Time Lord biodata. (AUDIO: Her Own Worst Enemy, Stolen Goods)

As the COLT-5000
Arriving at the Vesh Taralesh Emporium, she killed Vesh Taralesh when she refused to divulge information on Jenny's location. She made a deal with Garundel so that she could track Jenny across time and space, as long as she spared Garundel's life, and was intrigued by his information on Noah. (AUDIO: Stolen Goods)

COLT-5000 later tracked Jenny to Kamshassa, being given information by offering a reward. The newly liberated populace held her off long enough for Jenny and Noah to escape. (AUDIO: Neon Reign)

She caught up with them at the Eye of the Storm, a spacestation located in zero space. She forced the clone scientists aboard to help her capture Jenny and Noah by exploiting the clones' superstition that the death of the original scientists, upon whom the clones were based, would kill them and as an extra safety measure, the COLT-5000 also planted a wormhole bomb.

She caught Jenny and Noah and had them taken to the station’s cloning facilities, planning to clone them and sell their DNA to multiple buyers. Jenny persuaded the clones to turn on her, prompting her to detonate the bomb. As Jenny and Noah were using their ship, the Jenny One, to escape into a wormhole, the COLT-5000 attempted to break in after them. They successfully flew into it, closing it behind them, losing the COLT-5000 in the process. (AUDIO: Zero Space)

Having her timeline rewritten
Finding herself curious as to why the COLT-5000 was always chasing her and Noah, Jenny decided to use her new vortex manipulator to hop back along the COLT-5000's timeline and learn its origins. This took her to the planet Mirabilis 5, a neighbouring planet of New Damson. There, they learnt that philanthropist Seavus Colt had acquired the Cybermen's augmented assessment technology from New Damson after the fall of the Cyber-reign, and had set about using it for similar purposes.

Inhabitants of Mirabilis 5 were being assessed for their useful. Being deemed useless to society, Geraldine Diamond was converted into the COLT-5000. Realising that the COLT-5000 had no choice in her creation, Jenny took it into her own hands to save Geraldine and erase this timeline. Showing her the good she had done throughout her life, including the inspiration she had given to so many students, Geraldine retook her assessment and passed with flying colours. Arriving back in the future, Jenny was not confronted with the COLT-5000, but rather Geraldine Diamond. However, they were both horrified to learn that Noah had taken her place and was now the cyborg bounty-hunter that Geraldine had been destined to become. (AUDIO: Her Own Worst Enemy)

Behind the scenes
According to Sean Biggerstaff, Barnaby Edwards "went straight to Dame Siân Phillips" to cast COLT-5000. Before this casting was annouced, the cover for Jenny: The Doctor's Daughter showed COLT-5000 with a male actor.