Joseph (The Master)

god he's the Master in the doctor who au for some reason

The Master was plucked out of the Time War by the Forever to be used against the Doctor, however he escaped them and went into hiding as a human being using the pocket watch thingy. He was a valuable but somewhat eccentric inventor on the side of an army of rebels in a war against the Mago Vitrim. He helped them create game changing weapons, cloaking devices, and tracking systems, but spent his free time on building something he couldn't understand. Its design came to him in dreams, taking the form of a hulking mess of wires and metal in the corner of his lab.

The Doctor arrived at the behest of his frenemy Roran, who wanted revenge on the Mago Vitrim that were turning karnith into monsters (long story). The Doctor and Joseph got along very well, but his migraines and their accompanying moodswings worsened. The moment the Doctor realized that thing in the corner was a ramshackle TARDIS, Joseph opened the pocket watch, and once again became the Master.

The Master didn't realize the Doctor had effectively killed all of the other Time Lords, and he also didn't have the time to care. He was as cruel and selfish as ever, and remembered the plot that the Forever were weaving against his lifelong rival. The Master convinces the Doctor that he needs his help fighting the Forever--with the understanding that the end game will be the Master gaining the Forever's power.

So the Doctor stuffs the Master in a secret broom closet in the TARDIS and goes on quests at his command to get what he needs to build some sort of weapon.

The Master is nihilistic and empty inside. The pursuit of power and his constant struggle with the Doctor is essentially all he has. He still suffers from an ominous drumming in his mind, which drives him to extremes to keep it quiet.