- Isa Whitney – Opium addict and one of Watson's friends.
- Kate Whitney – Wife of Isa, friend of Mrs. Watson. It's on her behalf that Watson visits the opium den where he then finds Holmes in disguise.
- Mrs. Watson – Wife of our Watson.
- Neville St. Clair (alias Hugh Boone) – The man whose disappearance Holmes is investigating; goes in disguise as the hideous beggar Hugh Boone.
- Mrs. St. Clair – Wife of Neville, who is certain that her husband is still alive.
- Inspector Bradstreet – The local police officer who ushers Holmes in to see "Hugh Boone."
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