Speculation argues that HABIT is a demon, an angel, a monster, a ghost or a possessing force. Little is known about who or what exactly HABIT is, though clues have started to come to the forefront. One of the earliest clues came during the infamous "Twitter Fight" in which HABIT said "IT IS I WHO inHABITS THIS SANCTUARY. " making a strong case for HABIT as a spirit or ghost who can posess others. When asked directly who he was during a chat with Agnol HABIT said "I AM MANKIND'S BAD HABIT."
After the establishment of The Trials and HABIT's Twitter account other clues began to come out as HABIT began claiming either direct responsibility or involvement in various violent events around the world, like uprising in Egypt.
After HABIT assumed control of Damsel's blog, CanYouSeeTheWords, HABIT began implying that he had been, perhaps through possession, several of the world's worst serial killers and villains, including Vlad the Impaler, Albert Fish, Josef Mengele and Ed Gein. He also began posting first hand stories of modern encounters that he has had with victims. These stories began to place him in a modern context (for instance mentioning a song released in 2001.) They also start to reveal small facts, like the fact that he can control/posess one or more persons, and make them incredibly powerful.
HABIT's posts on CYSTW seem to reveal that he is working with Slenderman in some capacity.
As of so far in the series, it has (arguably) not been revealed or implied that HABIT has a true corporeal form.