Armand Christophe is a home-owner on the Imperial City Waterfront. He spends a lot of time with the seedier elements of Tamriel. Supposedly, Armand humiliated Guard Captain Hieronymus Lex years ago, and Lex has always had a personal vendetta with him because of the incident.
Armand Christophe is a member of the Thieves Guild. If bribed, beggars in the Imperial City will tell the Hero to seek out Armand Christophe for more information about the guild.
He meets prospective Thieves Guild members at around midnight in the Garden of Dareloth, located in the waterfront right behind Dareloth's house where he gives them an assignment to test their suitability for membership.
Before joining the Guild he may be repeatedly pickpocketed for lockpicks. This is a good way to level up sneak. No bounty will accrue and he won't report the crime. However, after joining, attempting to pickpocket him will result in suspension from the guild.