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.
The fan term for ones possessed by HABIT, is "inHABITed" and refers to HABIT possessed things as such.
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.
Though it had been suspected, and heavily hinted since the beginning, that HABIT and Evan were linked, it wasn't until late in the series that the link became more concrete. Whether HABIT is an alternate personality of Evan's, or a possessing spirit as he has always claimed, is unclear, but it has become clear that HABIT can assume control of Evan's body and cause him to act against the rest of the cast.
It was suggested that the times Evan was seen speaking to the empty chair (in Slender man and at the end of Ryan & the SEVENTRIALSOFHABIT) may be early instances of HABIT taking possession of Evan.
The possession seemed to grow stronger, and HABIT's personality started coming to the forefront in videos like May & June, where Evan/HABIT was instrumental in killing a deer with his bare hands. In the the video Box 6 it seemed clear that Evan was at least ocassionally being possessed by HABIT. His attitude towards Green Feathers, and the fact that he knew where the box had been hidden, were outside Evan's normal behavior. However, it soon became clear, with Jeff's kidnapping and disappearance (see videos Not dead yet and MOVING IN,) that Evan had been completely taken over by HABIT.
HABIT continued his possession of Evan, killing Daniel, torturing Steph and Jeff, and committing other crimes in videos like Next. Evan's mode of dress started to change as well, and started wearing a Hawaiian style shirt and straw fedora, (again, as seen in Next,) very different from the usual jeans and t-shirt style of Evan.