Krasko
From Tardis Wiki, the free Doctor Who reference
Krasko was a murderer from the 51st Century who was sent to, then released from the Stormcage Containment Facility. He murdered around 2000 people. Whenever he remembered what he'd done, he was delighted with the memory, though Stormcage had punished him by installing a neural implant that stopped him from trying to murder anyone, no matter how much he wanted to.
The Thirteenth Doctor, Graham, Ryan, and Yaz encountered him in Alabama 1955, after finding a suitcase of time displacement equipment. He wanted to change history by making Rosa Parks not go into the bus.
Krasko knew about TARDISes, as he recognised the Doctor's TARDIS as one.
