Marcus

A human male you encounter in the Desert along with a succubus named Lucia, carrying around a massive pair of balls in a wheelbarrow.

Marcus will ask you for a second opinion as to "stay human" or "go demon". Choosing either one of these options will have the following effects: Either choice will give you the chance to find  Succubi's Delight  in the Desert, but with different texts (depending on your choice).
 * "stay human" : Will lower the characters libido and corruption but will raise lust.
 * "go demon" : Will raise the characters corruption and lust.

A follow-up scene occurs after advising Marcus. If he stays human, you'll oversee Lucia "feeding" on Marcus' cum. If he becomes a demon, Lucia will fly in and tell you about it, showing you the Lethicite produced.

Trivia

 * Fenoxo has apparently stated that a Markus encouraged to "go demon" will show up in a refurbished Demon Factory, while one who was told to "stay human" will appear in the same place, but as a "sex cow", as a result of the furious Lucia betraying him.