The Mentor (Death to the Doctor!)

The Mentor was an overweight alien, described by the Third Doctor as a "second-rate impostor". The Mentor himself claimed that his name was not a copy, but "purely coincidental".

He encountered the Third Doctor, who remained unimpressed, and the Brigadier, who pulled a gun on him.

Later, he appeared at Research Base Truro, where he met with several other aliens the Doctor defeated in his various incarnations, who turned on each other as a result of their mutual suspicion following the accidental death of Kraarn of the Kraagaaron. The Mentor died in a three-way shoot-out with Bolog and Plink, two of the Doctor's other very minor enemies.

As with the others, when the Tenth Doctor arrived in Truro in the TARDIS, he did not recognise the body. (COMIC: Death to the Doctor!)

Behind the scenes
The figure the Mentor is unintentionally impersonating is left vague, but is strongly implied to be the Roger Delgado, whom he somewhat resembles and shares a similar name. He paraphrases the line "I am usually referred to as the Master," from the television story Terror of the Autons when introducing himself and later says "I am the Mentor and you will obey m-" before being cut off, a play on the Master's catchphrase. Script extracts of the comic printed in The Widow's Curse comic collection confirms this intention, describing the Mentor as a "low-rent Master".