Kirth Gersen is the protagonist of the five Demon Princes novels by Jack Vance, set approximately 1500 years in the future.
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During the events described in The Killing Machine, however, Gersen gains fabulous wealth (10 billion SVU in cash) by running a scam on Kokor Hekkus, a Demon Prince who is holding him for ransom, and on Interchange, the institution that facilities the ransoming of kidnap victims.
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In The Killing Machine, he encounters Alusz Iphigenia Eperje-Tokay, and seems to be growing involved with her; but early in the following book, The Palace of Love, the pair split up, chiefly because Alusz Iphigenia cannot understand his cold and singleminded pursuit of vengeance, especially since the now-wealthy Gersen could subcontract his dangerous and horrifying life's work.
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Finally, he has the aid and companionship of Alice Wroke in The Book of Dreams.
According to Kirth Gersen, their development on their homeworld, which they call "Ghnarumen" (Lambda Gruis III), was profoundly influenced by the artificial introduction of Neanderthal men in prehistory.