Krasko

Krasko was a serial killer from the far future who was sent to and then released from, the Stormcage Containment Facility. He murdered around 2000 people. He revelled in his crimes, though Stormcage punished him by installing a neural restrictor into his brain, which stopped him from inflicting harm or killing any living thing, no matter how much Krasko wanted to.

Using a vortex manipulator, he travelled back in time to change history.

The Thirteenth Doctor, Graham, Ryan and Yaz encountered Krasko in Alabama 1955, after finding a suitcase filled with time displacement equipment. He wanted to change history by preventing Rosa Parks from starting the Montgomery Bus Boycott.

Krasko's plot was averted by the Doctor and her companions who managed to ensure Rosa began the boycott. Krasko's vortex manipulator was destroyed by the Doctor, leaving him unable to time travel. Ryan later used Krasko's own temporal displacement weapon, as he attempted to block the bus that was on Rosa ’s path, to send Krasko as far back as he could set the device for, forever stranding him in the distant past. (TV: Rosa)

Personality
Krasko's racism led him to believe that "things started to go wrong" with Rosa Parks' victory. He told Ryan that "[his] kind would be put in [their] place". He showed no remorse for the two thousand people he had murdered and when recalling his crimes to the Doctor, he smiled with pleasure. When Krasko was recalling his crimes to the Doctor, he made a joke about not really remembering how many people he'd murdered.

Because of the restrictor in his brain stopping him from inflicting or even killing other people, Krasko was visibly frustrated when the Doctor snatched the Vortex Manipulator from his wrist and stomped on it, not being able to strangle her when he managed to overpower her briefly.

He also didn't care about the Laws of Time and what could happen if Rosa Parks did not make her historic victory.

He was able to blend in while in 1955, dressing in the appropriate fashion as not to raise suspicion.

He knew about TARDISes, as he recognised the Doctor's TARDIS as one and understood artron energy. (TV: Rosa)