The Monk (The Persistence of Memory)

One incarnation of the renegade Time Lord often dubbed the Meddling Monk, postdating the Great Time War, returned to primarily calling himself Mortimus. After an encounter with the Twelfth Doctor, he ended up sent away in his TARDIS with the memories of his scheme erased by a memory worm.

Biography
This incarnation of the Monk knew that Gallifrey had survived the events of the Last Great Time War and were "hiding themselves away" from the universe, (PROSE: The Persistence of Memory) something which had discovered shortly before his travels with. Before his regeneration, that Monk had voiced his intention to deal with the Doctor. (AUDIO: Divorced, Beheaded, Regenerated)

On one occasion, the Monk discovered an abandoned ship manned by robots and full of time scoop technology. He took the ship to Loch Ness in 1979 with plans to use the time scoop to depopulate the planet and sell it to an alien race. Before doing that, he used the scoop to bring a plesiosaurus to the loch, increasing the amount of Loch Ness Monster sightings and attracting attention to the loch.

The Twelfth Doctor and Clive Finch came to the loch to see the monster and discovered the Monk's plan. The Doctor used his sonic screwdriver to take control of the ship and had its robots restrain the Monk. The Doctor found his TARDIS, disguised as a plain white wardrobe, and decided it would be best to let Mortimus go. The Doctor retrieved a memory worm, erased Mortimus' knowledge of the plan, along with the rest of his short-term memory, and sent him away in his TARDIS. (PROSE: The Persistence of Memory)

Personality
This incarnation of the Monk, much like his predecessors, enjoyed meddling with history to suit his own selfish ends. Mortimus, in particular, was considerably more jovial than those that came before him, but his schemes lacked any attempt of moral justification. (PROSE: The Persistence of Memory)

He deemed the Twelfth Doctor a 'killjoy' whose insistence upon causality was 'naïve and boring'. (PROSE: The Persistence of Memory)

Appearance and clothing
In this incarnation, Mortimus was powerfully built. He had pronounced laughter lines and eyes that twinkled with humour, as though he found a lot to laugh about in life.

During his encounter with the Twelfth Doctor in Loch Ness wore a richly patterned scarlet robe and a paisley scarf for a belt, pairing this with a pair of sturdy shoes. (PROSE: The Persistence of Memory)

Behind the scenes

 * The Twelfth Doctor would have faced an earlier incarnation of the Monk on-screen (one that would precede the incarnation played by Peter Butterworth) in an ultimately unproduced TV story. How the Monk Got His Habit, a story pitched by Peter Harness for Series 9, would have seen the Monk (prospectively cast as Matt Berry) mistakenly drive Grigori Rasputin mad. The Doctor would have him regenerate into a duplicate of Rasputin, forced to live out the remainder of his lifetime; explaining that his habit of disguising as a monk began when he was forced to portray "the Mad Monk".
 * While the story was ultimately rejected, a prequel would later be released as part of Doctor Who: Lockdown! as an "unproduced novelisation", although it is not considered a valid source on this wiki.