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It was previously confirmed that former Coronation Street star Suranne Jones has been cast in the instalment as a young woman named Idris.
Gaiman apparently told reporters at the recent New Yorker festival that the character "may be an old friend of the Doctor's with a new face".
The writer also admitted that Doctor Who is part of his "internal landscape" and revealed that his script takes inspiration from Patrick Troughton's final story as the Doctor, first broadcast in 1969.
"At one point, I steal a plot from an old episode called 'The War Games'," he said.
Gaiman's episode is expected to air third in the next series.
Doctor Who will return to BBC One this Christmas.
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