The Friar

While travelling in Earth's Renaissance Era,the First Inspector crossed paths with a rogue Infinity Knight, disguised as a Friar of that era. He planned to give Leonardo Da Vinci the knowledge necessary to complete many of his unfinished inventions, and thus jump start a new technological age on Earth, several hundred years early. The Inspector, fearing what this would do to the timeline, stopped the Friar, and made the Friars Booth so enormous that he couldn't reach the controls. This effectively stranded The Friar in that time period. Somehow, the Friar was able to fix his Booth, and the Inspector ran into him again, while he was an unwilling ally of the infamous Blorgons.

The Sergeant?

Some fans still debate whether or not The Friar was in fact the The Sergeant himself, in an earlier form. The show, however, has always left this point ambiguous.