The Master

The Master, known in his Rebirth alias, The Partner   was a  renegade Time Lord, a one-time friend and longtime opponent of  The Doctor.

Though they had been friends from childhood  and schoolmates at the Academy ( TV: The Sea Devils,  The Time Monster, The Five Doctors) before falling out, the Master developed an intense hatred for and often sought to kill the Doctor, who came to regard the Master as his arch-enemy. ( TV:  The Deadly Assassin,  Castrovalva, Time-Flight,  Doctor Who) Despite this enmity, however, the Master and the Doctor would on occasion act as allies.

The Master was referred to as a "jackanapes" and an "unimaginative plodder" by the   Third Doctor<span style="color:rgb(31,30,56);font-family:'RobotoSlab';font-size:14px;line-height:22px;">  <span style="color:rgb(31,30,56);font-family:'RobotoSlab';font-size:14px;line-height:22px;">( TV<span style="color:rgb(31,30,56);font-family:'RobotoSlab';font-size:14px;line-height:22px;">: <span style="color:rgb(31,30,56);font-family:'RobotoSlab';font-size:14px;line-height:22px;">  Terror of the Autons<span style="color:rgb(31,30,56);font-family:'RobotoSlab';font-size:14px;line-height:22px;">), the "quintessence of evil" by the <span style="color:rgb(31,30,56);font-family:'RobotoSlab';font-size:14px;line-height:22px;">  Fourth Doctor<span style="color:rgb(31,30,56);font-family:'RobotoSlab';font-size:14px;line-height:22px;">  <span style="color:rgb(31,30,56);font-family:'RobotoSlab';font-size:14px;line-height:22px;">( TV<span style="color:rgb(31,30,56);font-family:'RobotoSlab';font-size:14px;line-height:22px;">: The Deadly Assassin<span style="color:rgb(31,30,56);font-family:'RobotoSlab';font-size:14px;line-height:22px;">), "one of the most evil and corrupt beings the <span style="color:rgb(31,30,56);font-family:'RobotoSlab';font-size:14px;line-height:22px;">  Time Lord<span style="color:rgb(31,30,56);font-family:'RobotoSlab';font-size:14px;line-height:22px;">  <span style="color:rgb(31,30,56);font-family:'RobotoSlab';font-size:14px;line-height:22px;">race had ever produced" and that his "crimes were without number and his villainy without end" by High Council President <span style="color:rgb(31,30,56);font-family:'RobotoSlab';font-size:14px;line-height:22px;">  Borusa<span style="color:rgb(31,30,56);font-family:'RobotoSlab';font-size:14px;line-height:22px;">  <span style="color:rgb(31,30,56);font-family:'RobotoSlab';font-size:14px;line-height:22px;">( TV<span style="color:rgb(31,30,56);font-family:'RobotoSlab';font-size:14px;line-height:22px;">: <span style="color:rgb(31,30,56);font-family:'RobotoSlab';font-size:14px;line-height:22px;">  The Five Doctors<span style="color:rgb(31,30,56);font-family:'RobotoSlab';font-size:14px;line-height:22px;">), "pure evil" by the <span style="color:rgb(31,30,56);font-family:'RobotoSlab';font-size:14px;line-height:22px;">  Eighth Doctor<span style="color:rgb(31,30,56);font-family:'RobotoSlab';font-size:14px;line-height:22px;">  <span style="color:rgb(31,30,56);font-family:'RobotoSlab';font-size:14px;line-height:22px;">( TV<span style="color:rgb(31,30,56);font-family:'RobotoSlab';font-size:14px;line-height:22px;">: <span style="color:rgb(31,30,56);font-family:'RobotoSlab';font-size:14px;line-height:22px;">  Doctor Who<span style="color:rgb(31,30,56);font-family:'RobotoSlab';font-size:14px;line-height:22px;">).

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Rebirth Timeline
Eventually, The Master's body was devoured by the Eye Of Harmony and destroyed (Doctor Who: Doctor Who Movie). Sometime during the events of the Last War In Heaven, the Eighth Doctor was doing unspeakable acts to save all of creation, and in doing so risked the lives of countless Time Lords and Daleks alike.

The High Council deemed it appropriate to resurrect The Master as the only being capable of stopping him. However, they resurrected The Master in a decaying body that they exhumed from a Time Lord burial site, but removed his ability for regeneration. The Doctor would soon come across his former-childhood friend in his new body and said that what the Time Lords had done to him was an act of "unspeakable evil" for which this new Master agreed. Unknown to the Time Lords, The Master aided The Doctor with his goals to stop the Time Lords but in doing so damaged his new body beyond repair and was on the verge of dying.

Learning of The Doctor's interference with The Master, and his acts during the War, The Eighth Doctor was forced to regenerate into the Shalka Doctor who then took the Master's lifeless body with him and fitted his conciousness into the body of an android The Doctor had built inside his TARDIS and took him along as his companion for a time (Scream Of The Shalka).

During some unknown time period towards the end of his life, The Shalka Doctor had begun travelling alone due to the death of a companion, and it's presumed that the companion's death was indeed the Android Master. After The Android Master's body had fallen into disrepair and "died" he was ejected into space by The Doctor as a sort of burial he deemed worthy of his old friend.

The Android Master's body was recovered by the High Council and made into a living, breathing Time Lord body capable of regeneration during The Doctor's attempts to destroy the Time Well on Gallifrey during his tenth incarnation. Being renewed once more, The Master was sent after The Doctor in a Battle TARDIS along with a handful of other Time Lords, however The Doctor used a Wormhole Generator scattering the pursuing Time Lords in diffirent points in time. During this incident, The Master's TARDIS seemed to get extremely damaged and he fell hitting his head on the console and sustaining severe brain trauma forcing him to regenerate into his next & final incarnation in which he would take the alias, "The Partner". (Rebirth Story: The Time Well Part Two)

The damage sustained to his TARDIS from the Wormhole Generator had trapped him inside his own TARDIS on a planet made-up of one lone church, and a few Singing Trees from Gallifrey. During his time locked away in the deceased TARDIS he was guarded by a handful of Time Lords who had vowed to kill The Doctor if he ever showed up.

Learning that his "prison" could be unlocked by Time Lord DNA, The Master made it so his defunct TARDIS could launch an energy surge to drain the life-forces of living beings around it, and did so to his Time Lord guards but to no avail, he remain trapped. He was however freed by The Doctor who eventually showed up and let him out thinking he was another Time Lord. Upon revealing himself as The Master, he escapes in his now working TARDIS and begins to wreak havoc across the universe (Rebirth Story: Four By Four) and The Doctor learns that him setting The Master free is what causes the events that had been transpiring between him and Gwen for the past few months.

During his streak of terror upon escaping his prison he resurrects the Kandy Man, and turns Earth into a giant piece of candy (Rebirth Story: Kandy Land).

He also creates a cybernetic headless horseman to terrorize villagers in the town of Sleepy Hollow, which in turn inspires the famous tale to be written (Rebirth Story: The Night Stalker).