Duncan was a human who lived in Scotland in the 18th century. He was a friend or relative of Seamus MacIntyre.

In 1746, Duncan was a member of the Scottish forces in the Jacobite rebellion. He and his comrades came to the rescue of Seamus, the time-lost Montgomery Scott and Seamus' future wife Megan when they were trapped in a cabin by English officers, all of whom were killed by Duncan and his group. Seamus, Megan and Scott then accompanied Duncan to Inverness. (TOS novel: Home is the Hunter)

Duncan's precise relationship to Seamus is never specified.