No one knows how to hurt The Doctor better than The Master, no matter their form or alias at the time.
But all you really need to know is that The Doctor and The Master are the Batman and Joker of the Doctor Who history.
Another rogue Time Lord, The Master uses their abilities in the name of chaos and evil, goading The Doctor into several conflicts over the course of Doctor Who history.
As if Daleks and Cybermen weren’t enough of a pain, The Doctor has their own version of Moriarty to go along with their Sherlock Holmes: The Master!