The Master

A version of the Master once attempted to destroy the Doctor on the planet Tersurus.

Biography
When spying on the Doctor, the Master had inadvertently enabled two way communication between the two, and unwittingly revealed several of his plans to eliminate the Doctor. When watching the Doctor on his TARDIS monitor, he accidentally activated the communication system, and even when he attempted to terminate the connection he only turned the picture off while leaving the sound active. After the two had arranged to meet on Tersurus, it became apparent that the Master had travelled back in time to bribe a Tersuran architect to install several traps in the castle they were now meeting. Unbeknownst to him however, the Doctor had also done the same, bribing the same architect to change the details of the traps. This eventually led to the Master floundering around in the sewers of Tersurus for centuries, after which he was forced to ally himself with the Daleks as they were the only life-forms without noses and hence the only creatures who would be willing to work with him.

As part of this alliance, the Master had himself "augmented" with Dalek technology, resulting in not only a more youthful appearance, but also "Dalek bumps" installed on his chest. Although he remained adamant they were etheric beam locators, the Doctor continually teased the Master due to the fact that they resembled breasts.

When he believed the Doctor to be dead, the Master renounced his evil ways, before the Doctor regenerated into a female form. With both Time Lords finding themselves attracted to each other, they walked off, arm in arm. (DW: The Curse of Fatal Death)

Behind the scenes

 * Like The Curse of Fatal Death, this version of the Master was not considered canon. However, EDA: The Tomorrow Windows later brought him into canon.
 * He has been called "The 17th Master" by some fans, based upon a behind-the-scenes featurette on the making of the spoof. In the special itself he is not referred to as anything else but the Master.