The Demon in Your Head is a strange and malevolent spirit that haunted many people in the story, driving them insane before possessing another victim via literally jumping into their mouth : the tiny demon was unable to be seen or sensed by victims until they had already become possessed.
The Demon in Your Head succeeded in travelling from Canada to the United States via one such host and ultimately hopped from them into the psychiatrist, who had believed that they had "cured" the madness. And only for it to be revealed that the Demon was now active inside him and presumably getting ready to repeat the cycle endlessly.
Powers and Abilities
The Demon in Your Head is a symbolic entity, representing madness - victims of its possession become unable to sleep, think clearly or breathe well : eventually they become completely insane and can do nothing but run in terror.
The demon enters victims via literally jumping into their mouths, being extremely tiny (as well as invisible to the human eye) - it is also very likely intangible too, since it does not choke or otherwise physically harm those it enters (though it does mentally scar them).
The demon is presumably completely immortal as well, since it is a malevolent spirit and symbolic of madness - meaning that even if banished the demon will find some way to exist, for as long as madness itself exists.
- While the origins of the Demon in Your Head predates modern Marvel, the entity, being a demonic embodiment of madness, could have links to Dusk - a modern age Marvel villain who is also a demonic embodiment of madness (and a rival of Nightmare, another demon that is fond of possessing and tormenting humanity).
- The Demon in Your Head's origins in Canada would also place the spirit within the realms of Llan the Sorcerer or the Great Beasts, though it is also entirely probably the demon has no links to any but itself.