Kiv

Kiv was the leader of the Mentors and the direct superior of Sil during the late 24th century.

Biography
Kiv helped the Mentors become rich from profitable deals, and was worshipped by many members of his race, particularly Sil. He was involved in weapons dealing with the primitive peoples of the planet Krontep in 2379. (TV: Mindwarp)

Needing a brain transplant
Kiv's great intelligence and business expertise were likely due to his mutated brain, which was vastly larger than the brains of most other Mentors. However, Kiv's brain did not stop growing larger as he got older, and later in his life, his brain was compressed against his skull, which caused him severe pain and would have eventually resulted in his death. To prevent this, he hired the scientist Crozier to find a way of transferring his brain into a host who could hold his expanding brain without trouble.

The first operation performed by Crozier moved his brain into the body of a similar Mentor, albeit tan-coloured and with a stinging barb. Kiv's second body held the brain for a bit longer but was not a perfect solution - not least due to the fact that the mind of its original owner, a fisherman, kept influencing Kiv's business decisions. (TV: Mindwarp)

Various fallouts of taking Peri Brown
According to the Matrix, when the brain started getting too large for the second body, Crozier tried a different technique and transferred Kiv's consciousness directly into the brain of Peri Brown, destroying her original mind and removing the problem of Kiv's constantly expanding brain. Shortly afterwards, however, King Yrcanos, who had fallen in love with Peri, broke into Crozier's laboratory. Discovering what had happened to her, Yrcanos apparently killed everyone there, starting with Kiv. (TV: Mindwarp)

However, due to the temporal manipulations of various competing factions of Time Lords, a number of varied outcomes emerged on Kiv's taking of Peri's body, due to some Time Lords likened the fate to an assassination to remove a potential witness who could contradict the altered recordings played at the Doctor's trial.

Many accounts at least agreed that the sympathetic Time Lords pulled Peri out of her timestream long enough for Yrcanos to prevent Crozier from placing Kiv in her body, (AUDIO: Peri and the Piscon Paradox) one detailed that the transfer was a complete failure, (AUDIO: Antidote to Oblivion) with another explaining that the mind transference failed because Crozier's earlier interrogation of the Doctor had allowed Mandrake to escape the Doctor's mind and enter Crozier's computer, and that it was actually Mandrake's consciousness that was transferred into Peri's body instead of Kiv. (AUDIO: The Widow's Assassin)

Another account suggested that Kiv survived Yrcanos's attack, and continued to possess Peri until a new host body was needed. (PROSE: Sil and the Devil Seeds of Arodor)

In Adam Carter's body
Kiv eventually ended up in the body of a human lawyer called Adam Carter. Keeping his true identity secret from everyone but Sil, he defended his old associate when he stood trial for introducing a dangerous drug to Eurozone. (HOMEVID: Sil and the Devil Seeds of Arodor)