Kotris

Straxus was a Time Lord who worked for the High Council of Gallifrey.

Biography
Straxus was sent by the High Council to the Doctor's TARDIS. He ordered the Eighth Doctor to Lonsis in 2006 to retrieve Lucie Miller from a temporal blackspot. The Doctor agreed on condition that Straxus fix his TARDIS. Straxus agreed and gave the Doctor a time ring for his mission.

On the Doctor's return from his mission, Straxus told the Doctor why Lucie was originally taken out of her own time and sent to the Doctor. Straxus then helped the Doctor defeat the Cybermen on Lonsis before returning to Gallifrey. (AUDIO: Human Resources)

Straxus was later sent by the Time Lords to take the Doctor back to Gallifrey, where he was to be quarantined with the rest of the Time Lords in order to prevent the resurrection of Morbius. (AUDIO: Sisters of the Flame) Straxus failed to find the Doctor in time and instead was captured by Cristophe Zarodnix and was forced to surrender his DNA in order to resurrect Morbius. Straxus was imprisoned for ten years while Morbius fed of Straxus' life energy whenever he became weak. The Doctor and Lucie came for Straxus and the Doctor sacrificed his life to save the universe from Morbius. Straxus' life energy was restored after the Time Lords corrected history. Straxus was taken back to Karn, where he offered to take Lucie home to Earth. (AUDIO: The Vengeance of Morbius)

In one incarnation, Straxus went on a mission for the Time Lords to 19th century Earth. He became trapped there and decided to place ancient historical treasures in a market place where they were never meant to be. By doing this he would reshape human history, which would in turn inform the Time Lords as to which time period he was trapped in. During this time, an attempt was made on his life by Septimus, one of his Cwejen clones. (AUDIO: The Adventure of the Diogenes Damsel)

Following another regeneration, Straxus was ordered by the Lord President of Gallifrey to send the Doctor to France during World War I. (AUDIO: The Great War)

Straxus later saved the Doctor and his new companion Molly O'Sullivan from a group of Daleks. His TARDIS was attacked by the Daleks and he escaped back to Gallifrey. (AUDIO: Tangled Web)

In his next incarnation, Straxus' DNA was altered by the Daleks and he became known as Kotris. Kotris spent centuries on the planet Srangor helping the Daleks create a temporal chamber. Kotris was erased from history when his earlier self, seeing what he would become, was exterminated by the Daleks. (AUDIO: X and the Daleks)

Behind the scenes
On the 12 July 2011 edition of The Big Finish Podcast, Nicholas Briggs revealed that the original actor hired to play Straxus had to be fired. When the original actor proved unsuitable for the role, Briggs quickly called someone he knew — Nickolas Grace — to take over. The crew became so enamoured of his performance on AUDIO: Human Resources that they wrote Straxus into The Vengeance of Morbius. Briggs went on to mention that this was the only time in Big Finish's Doctor Who production where an actor had to be fired for performance deficiencies. It's unclear whether Briggs was exaggerating, but it is probably fair to say that such dismissals are at least highly unusual at Big Finish.