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- Dr. Mahmoud (a.k.a. "Stinky") – Stranger in a Strange Land
- Margrethe – Job: A Comedy of Justice
- Dr. Mary Lou Martin – Let There Be Light
- Mary (aka "Allucquere") – The Puppet Masters
- Helen Mayberry – Time Enough for Love
- Gloria McNye – Delilah and the Space Rigger
- Memtok – Farnham's Freehold
- Minerva – intelligent computer used to run the planet Secundus – Time Enough for Love, The Cat Who Walks Through Walls
- Miriam – Stranger in a Strange Land
- Frank Mitsui – Sixth Column
- Mort the Wart – The Moon Is a Harsh Mistress
- Mycroft – see Mycroft "Mike" Holmes, above
- Mother Thing – Have Space Suit—Will Travel
Read more about this topic: List Of Robert A. Heinlein Characters