Anyone who considers themselves to be a proper fan of Doctor Who knows that the Doctor's worst enemy isn't the Daleks, but the Master, a fellow Time Lord who essentially lives to torture the Doctor. For such a vital character of the show, the Master actually made their debut eight years after Doctor Who began.
Since the renegade Time Lord's first appearance, the Master and the Doctor have been locked in a seemingly endless war to best the other. However, they weren't always bitter rivals - they were childhood friends, and this can often be seen in the subtext of all their confrontations.
Hated - When The Master Took Over The Earth
The Master's dislike of the Doctor has been well established in Doctor Who canon, and they will do anything they can to hurt the Doctor, no matter how extreme. For whatever reason, the Doctor has always been very fond of the Earth.
Unfortunately, the Master has used this little nugget of information frequently over the last few decades for his own evil ends. In the 2007 season finale, the Master even managed to conquer the planet completely, devastating the Doctor and his companions.
Liked - When The Master Sacrificed Himself To Save The Doctor
The Master has never been one for completely selfless acts, let alone willingly dying to save his nemesis. However, that's exactly what happened in the Tenth Doctor's final episode.
Before the Master's sudden change of heart, he had challenged and beaten the Doctor. The whole of the human race had become the Master race, and he had succeeded in bringing the Time Lords back to life. However, when Rassilon betrayed the Master, the latter responded by sacrificing himself to save the Doctor.
Hated - When The Master Aged The Doctor
Despite the Master's tendency to target the Earth in his bid to hurt the Doctor, there are occasions where he tortures the Time Lord directly. One of the most shocking instances of this occurred during the penultimate episode of season 3.
Earlier in the season, the Doctor and Martha had encountered de-aging technology. The Master then reversed the process and turned it on the Doctor, ageing him 100 years, and then suspending his ability to regenerate, turning the Doctor into a Gollum-like creature.
Liked - When Missy Gave The Doctor An Army
Although John Simm's portrayal of the Master was absolutely brilliant, his successor, Michelle Gomez, completely nailed the role. Gomez's take on the Master (now known as Missy) brought unseen depth to the character, as well as being totally insane.
Despite very much remaining evil and cruel, this incarnation of the Master simply wished for her and the Doctor to be friends again. However, Missy wanted the Doctor to be more like her and so she gifted him with an army of Cybermen to conquer the universe.
Hated - When The Doctor Almost Killed Missy
Whenever the Master is involved, the Doctor always appears to be unable to stop them for good. However, he has come close a couple of times. After Missy's Cybermen army had been destroyed in season 8, the Doctor made to kill her to save Clara's soul.
When Missy soon returned in the first couple of episodes of season 9, she and the Doctor were allied for a while. However, this ended when Missy tried to trick the Doctor into killing Clara. As punishment, the Doctor abandoned Missy in an exploding city full of Daleks.
Liked - When The Doctor Gave Missy His Confession Dial
According to Gallifreyan tradition, a dying Time Lord gives their Confession Dial to their closest friend before they die. In the ninth season premiere, the Doctor had gone missing, forcing Clara and Missy to team up to find him.
During this allegiance, Missy revealed that the Doctor had given her his Confession Dial, shocking Clara and the viewers. This was one of many hints that the Doctor and the Master were more than just rivals, as they have both known each other for a very, very long time.
Hated - When The Master Refused To Work With The Doctor
The Doctor and the Master have teamed up almost as many times as they've fought, but their partnerships have always been full of mistrust and doubt. Usually, it's because the Master's audacious plans go wrong, and they have to stop them from destroying the universe.
In this particular instance, the Doctor, the Master and Missy were stuck onboard a ship full of Cybermen. In order to save a group of innocent humans, the Doctor asked for the Master's help, to which the latter took great pleasure in cruelly declining.
Liked - When Missy Became Good
This particular storyline is widely considered to be one of the Master's best in the series. In season 10, it was revealed that Missy was trapped in a Vault guarded by the Doctor, who had saved Missy from execution on the condition that she change her ways.
At first, everyone doubted that it would work, including the two fellow Time Lords. However, over the course of the season, Missy began to repent. She cried for all her victims, she helped save the Earth from the Monks, and she even made to stop the Cybermen.
Hated - When The Master Destroyed Gallifrey
Conquering and enslaving the Earth is one thing, but to conquer Gallifrey itself was a whole new level of spite and evil. The latest incarnation of the Master had lost all love for the Doctor after he learned that he only existed because of her.
Knowing that the whole foundation of Gallifrey was built on a lie, the Master enacted his ultimate revenge and savaged the planet that the Doctor had fought so hard to save. He then used all the dead Gallifreyans to make a new race of Cybermen, which he christened 'CyberMasters.'
Liked - When The Doctor Forgave The Master
No matter how many times the Doctor and the Master fight, the former will always come out on top. The two Time Lords are incredibly even matched but the Doctor has one small advantage: their comion.
In season 3, the Master took control of Great Britain, manipulated Martha's family, destroyed the Earth, and tortured the Doctor and Jack Harkness for a year. Despite all that, when the Doctor finally defeated him, he forgave the Master and offered to travel with him in the TARDIS, proving that there is an underlying friendship.