Wheezing Demon

The Man Who Can't Breathe, also known as the Wheezing Demon is a very dangerous and malevolent entity from The Further and is the main antagonist of Insidious Chapter 3.

Insidious: Chapter 3
In the movie, He heard Quinn's voice as she tried to contact her dead mother. He then began to terrorize Quinn. He later brings her to The Further, feeding on her soul. Elise, Specs, Tucker, and Quinn's father, Sean, try to stop him and rescue Quinn. The Wheezing Demon is known for possessing his victims, usually forcing them to commit suicide in order to take them away into the darkness forever. He briefly possesses Quinn and tries to make her kill herself with a box cutter. But Sean, Tucker, and Specs are able to restrain Quinn long enough for Elise to arrive and initiate a séance.

His goal is to drive souls to suicide and keep them in the dark as his "pets". He tries to drive Elise to suicide by pretending to be her late husband Jack (who had killed himself), and asking her to join him. Fortunately, Elise realizes that Jack would never ask her to do such a thing and attacks the demon. Elise bravely battles The Wheezing Demon and manages to get Quinn back to her body. But the demon catches Quinn just before she can return. In the physical plane, Elise and the others join hands with Quinn to mentally support her and aid her soul in defeating The Wheezing Demon alone.

Lilith, Quinn's mother, makes an appearance thanks to Elise's calls for aid and helps her daughter stand up to the demon. In The Further, Quinn regains her senses and quickly rips the respirator off of the demon's mouth, rendering him unable to breathe and destroying him once and for all.

Appearance
The Wheezing Demon wears a breathing mask and a dirtied, frazzled hospital dress. He's thin and emaciated, almost bald, with a wiry and deceptively weak-looking body. He has pale blue eyes, his skin is rotten and his feet are seemingly covered in a thick layer of tar, which strongly confirms his presence by the black footsteps he leaves in his wake. Under his breathing mask he's revealed to have a lipless mouth, which looks like a gaping hole.

Despite his horrifying appearance, he can shape-shift into more appealing people, such as Elise's deceased husband, in order to trick her into committing suicide.

He is also known to be making wheezing sounds, which explains his other name.

His appearance suggests that he was a man who died of some kind of respiratory illness (hence the breathing mask and the gown).

Personality
Much like the other antagonists, The Wheezing Demon is a vicious and corrupted entity that has some control over a part of The Further, specifically The Further version of the apartment complex that The Brenner family lives in. He is pernicious in a less brutal way than The Fire-faced Demon since according to Elise, he's a different type of entity. As a pose to many spirits in The Further, who desire to return to the realm of the living, The Wheezing Demon prefers to drag living souls into The Further for him to toy with and feed on.

He is clearly powerful, as he is shown to easily be capable of throwing Quinn out of her bed, climbing walls, shape-shifting, inconspicuously removing objects from people's body's, invading minds, and creating illusions in the living realm. The Wheezing Demon also seems to possess great power within the further too, being able to telekinetically move objects, and easily throw humans through the air (an impressive feat, as it is implied that resolute humans are stronger than a majority of the spirits in The Further).

It's worth mentioning that Quinn isn't his first victim. It's likely he's claimed many victims in the past, as Elise claims he's resided in the complex for a very long time.

Trivia

 * Elise refers to him as a 'demon'. However, his appearance is somewhat more human compared to The Fire-faced Demon or that of The Key Bearer, whose appearances are more monstrous. Making it possible that he is in fact a more powerful human ghost and not a true demon. However, this could simply be an expression on Elise's part, as it is a common tendency for people to use the term when referring to a malevolent, otherworldly entity.
 * The Wheezing Demon seems to be the only malevolent entity who has a particular fatal weakness, as The Bride in Black needed to be defeated by resolving his lingering emotional pain, and The Fire-faced Demon couldn't be destroyed at all.
 * The hospital gown and respirator suggest he may have been hospitalized before his death. It is possible he suffered from a terminal illness (perhaps a respiratory condition) or was on life support. The latter theory can be further supported by the scene where Sean Brenner finds his footprints leading to a broken window and sees his body lying against the pavement on the ground many stories below. This may indicate that while he was still alive, he may have grown tired of such a fragile state of life and committed suicide to end his pain. This theory can be further supported by his conversation with Elise while pretending to be Jack to trick her into joining him in The Further when he encourages her to "stop breathing in it, like I did".
 * If that is indeed how he came to be a spirit in the further, then he shares the same cause of death as Parker Crane, who also jumped to his death after being hospitalized.
 * The Wheezing Demon is the only antagonist who has not appeared once in the previous movies or has a connection to the Lambert family.
 * The Wheezing Demon is the second entity to fight Elise onscreen; the first being Parker Crane.
 * The Wheezing Demon is shown to be more clever than The Fire-faced Demon, as he poses as Lilith Brenner to lull her into a welcoming state of mind. Interestingly enough, when trying to prevent the other half of Quinn's soul from returning to her body, he caresses it lovingly. Although this may only be a ruse to deceive her soul into thinking it is loved so it will want to stay in The Further willingly.
 * The Wheezing Demon is the first entityto haunt Elise in her house. However, The Fire-faced Demon briefly appeared behind her in the last scene of the movie.
 * It can be argued that The Wheezing Demon is the only antagonist who succeeded in its goal, as it DID manage to drive many victims to their death and drag their souls into The Further. However, it can also be argued that both The Fire-faced Demon and The Bride in Black were successful (albeit temporarily). The Fire-faced Demon DID succeed in possessing Dalton Lambert and preventing his family from casting him out. Similarly, The Bride in Black succeeded in taking over Josh Lambert's body and living his life for a time. The Woman in White also succeeded in making Parker act like a woman and convincing him to murder countless victims, including the other members of The Lambert Family (Although Parker was unsuccessful in actually killing them).
 * Writer/Director Leigh Whannell told casting that he wanted The Wheezing Demon to look like the Sloth victim from the Brad Pitt movie, Se7en. The same actor, Michael Reid MacKay, was cast for the role.