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 kill 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}. Krasko was likely a psychopath, as he showed no remorse for the two thousand people he had murdered. Indeed, when recalling his crimes to the Doctor, he smiled with pleasure, hinting he was sadistic.

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