The Man Who Can't Breathe

The Man Who Can't Breathe, sometimes called the Man Who Lives in the Vents or the Wheezing Demon, is the main antagonist of Insidious: Chapter 3. He is the demon who haunts Quinn Brenner in Insidious: Chapter 3. He appears as a thin man with corpse-like skin in a hospital gown. He wears a breathing mask that covers his grimy and deformed mouth.

History
The Man Who Can't Breathe is first introduced when Quinn tries to contact her recently dead mother. He repeatedly terrorizes Quinn shortly after this; first distracting her from oncoming traffic which results in her breaking her legs, after which he pushes her off of her bed several times and tries to kill her by dragging her out into the window. The Man Who Can't Breathe lives in the apartment above the Brenner's which is supposed to be abandoned. He is able to possess people, which he will typically do to make his victims commit suicide. He does this because he wants to keep these souls as his "pets". He can also shape-shift into victims' loved ones. He tried to make Elise kill herself by shape-shifting into her deceased husband, Jack. After a tearful reunion, Elise realises it was the demon and slashes at the demon with a knife given by it. Elise easily beats the demon, helping Quinn and her get back to their body's. He then comes out of nowhere and pushes Elise back and grabs Quinn. Elise gets mad of this and she stomps on the ground making the demon fall into a hole. As soon as Elise gets back in her body, she tells Quinn's father and Specs and Tucker that the demon has Quinn's further body that she had to fight this battle alone.

Final Battle and Demise
Quinn's mother helps her by guiding her back to her body. She kills the demon by ripping off his mask that makes him breathe, making him vomit up ashes and thus killing him.

Trivia

 * The creator of Insidious, Leigh Whannell, says the Man Who Can't Breathe is a manifestation of physical misery, specifically cancer.
 * The Man Who Can't Breathe is played by actor Michael Reid McKay who is known for his role is the psychological thriller movie Se7en, were he played Victor, the Sloth Victim.
 * He is the first demon to die in the series.