Imaginary Enemy

Darien Fawkes: C'mon, didn't you have an invisible friend as a kid?

Bobby Hobbes: I had invisible enemies.

—The Invisible Man, "Ralph"

The Imaginary Friend is a common trope in fiction, in which it bears little resemblance to the counterparts in Real Life (after all, that would be boring, like real amnesia). Imaginary friends in fiction can be holograms, spiritual projections, ghosts, psychological conditions, stress or injury related psychosomatic conditions, even split personalities. And because it is more entertaining, sometimes they aren't very friendly, or are actively malign towards their host/audience.

Often they serve as a plot device to drive the person crazy, torment them, tempt them, or subvert their beliefs and actions in some way. Sometimes representing deceased people or perverted versions of people known by the victim, such as murder victims. Examples are not quite the level of Enemy Within, in that they do very little to actively harm the host and rarely exert any kind of physical control. Also, the host most often will not tell anyone, or at least keep the information with a select few, knowing that they won't be believed (insane asylums are no place to be alone with your imaginary enemy).

Sometimes, Your Mind Makes It Real, and the more you believe in the enemy and the more strongly you fight it, the more dangerous it becomes. See also The Killer in Me, Enemy Within and Enemy Without.

Examples of Imaginary Enemy include:

Anime

In ×××HOLiC, Watanuki thinks Domeki is trying to take his crush Himuwari for himself and constantly tries to stop him. In reality, there's nothing going on between the two.
In Soul Eater, Stein sees some visions of Not Quite Dead villain Medusa, eventually leading to a temporary Face Heel Turn. Filled with Foe Yay.
A major plot point in Higurashi no Naku Koro ni. The first arc reveals that when Rena wasn't living in Hinamizawa, she reported footsteps behind her and thought someone (or rather, Oyashiro-sama) was watching her all the time. Paranoia of this sort is actually a symptom of the Hate Plague, which can also make innocents seem creepy/trying to kill you. Occasionally subverted, though-- that shadowy presence watching people is usually Hanyuu (Rena was right!).

Comic Books

In Doom Patrol, Dorothy Spinner's main power is that she has imaginary friends who can affect the world in some very non-imaginary ways. Unfortunately, that includes The Candlemaker, who's not a nice person...
X-Man Sage developes a split personality named Diana Fox, who repeatedly fought with her and tried to take over her body.
Legion, of X-men fame, has made a number of imaginary people to keep him company and take away the pain and confusion. The only problm is his mutant power not only makes them real, but also gives them superpowers. Most of them are not nice people.

Film

In the film Mr. Brooks, Earl Brooks is both driven and tormented by his alter ego Marshall.
Fight Club has the narrator being manipulated and ultimately tortured by Tyler Durden, a Split Personality formed by his subconscious trying to get him to live his life. Ultimately he destroys Tyler by shooting himself (nonfatally) in the head, proving he can accept his own death.
In The Seven Percent Solution it turns out that Professor Moriarty being an evil criminal mastermind is a drug induced fantasy of Sherlock Holmes's.
In Secret Window it turns out that the killer is a cross between this and a Split Personality of the main character.
Dr. Caligari.
Drop Dead Fred is about a grown woman's childhood imaginary friend coming to life. He means well, and comes through for her in the end, but until then he just serves to make her already awful life that much more stressful.
The Red Knight that torments Parry in The Fisher King.
The scary-looking person behind Winkie's in Mulholland Drive.

Literature

In The Wheel of Time series Rand al'Thor is driven to distraction by the voice of an insane Lews Therin Telamon in his head, bordering on becoming an Enemy Within at times.
- Of course, Lews Therin sometimes argues that Rand is the imaginary one. These are Lews Therin's more lucid moments.
In multiple books of The Dresden Files, wizard Harry Dresden has regular visits from a fallen angel, Lasciel, after Harry briefly touched a Roman denarian coin inhabited by Lasciel. Only Harry can see the fallen angel, and she appears to Harry to tempt him and offer him power. In the book Dead Beat, Harry has periodic encounters with another character, and only late in the story does Harry realize that the character is actually Lasciel and only he can see her.
Appears in The Musicians by Andrzej Sapkowski when humans try to be bastards too hard for their own good.
Peter Quint in Henry James's The Turn of the Screw.
- He might not be entirely imaginary...
In Mistborn, Vin always hears the memories of her long-gone brother Reen telling her she can't trust anybody. It turns out that the voice she hears is neither imaginary nor her brother.

Live Action TV

Gaius' fantasy Six in the new Battlestar Galactica series. Strangely, Six later begins seeing an imaginary Baltar. And Baltar begins seeing an imaginary himself.
- And then it turns out that they were just "angels" trying to prod humans and cylons down a path that wouldn't lead to Eternal Recurrance.
In Farscape, John comes close to being driven crazy by a mental clone of Scorpius (whom he names "Harvey"), on behalf of a chip in his head. Towards the end of the series the clone becomes more of a confidante and sounding board than anything else, however.
Several characters are tormented or assisted in Buffy the Vampire Slayer by the First Evil.
Near the end of season 5 of Highlander the Series, Duncan is tormented by Ahriman, a demon only he can see, who likes taking the form of old enemies Duncan has killed. In the Season Finale he ends up confusing Duncan to the point of accidentally killing his friend and pupil Richie Ryan, causing a "What Now?" Ending ending. In the next season He faces Ahriman alone, and ends up defeating him through a combination of conversation and poking ballet.
Star Trek: Enterprise. In the Mirror Universe episode Jonathan Archer discovers that his counterpart in the other universe is a famous starship captain, and for the rest of the episode gets taunted by his imaginary counterpart, who constantly implies that he'll never amount to anything.
In the Monty Python's Flying Circus sketch about the Piranha Brothers, Dinnsdale is haunted by Spiny Norman, an giant imaginary hedgehog, who was normally twelve feet long, but could be up to eight hundred yards long if Dinnsdale was depressed.

Newspaper Comics

Calvin is known for his (sort of) imaginary friend, but his imagination is enormous enough to accommodate other characters as well. The monsters under his bed, while not specific recognisable individuals, are collectively an imaginary enemy to him. They are always trying to eat him, but are harmless in the sense that it's inconceivable to think of them ever succeeding. His mother once suggests to him that they're a case of Your Mind Makes It Real (or at least that's how he understands her saying they're imaginary), but just because he tried to stop believing in them didn't mean they stopped believing in him.

Tabletop RPG

One of main uses for illusion spells in Dungeons & Dragons. Turned Up to Eleven with Phantasmal Killer that causes target to face its worst nightmare and die of horror on the spot. For extra fun, on the Ethereal illusion/phantasm effects has a chance to be permanently materialized, and this includes monsters (now free-willed). Including Phantasmal Killer spells, of course.

Video Games

Manhunt 2 has Danny implanting Serial Killer personality Leo into his head for his career, but the Pickman Bridge malfunctioned, explaining of how he could see and talk to him. Ultimately, depending on which ending, it is either Danny or Leo would win the fight.

Web Comics

Zimmy from Gunnerkrigg Court sees monsters and other "things that ain't supposed to be there". If left untreated, her hallucinations become real.
Haley has dozens of these: every aspect of herself from self-doubt to desire-for-comedy to latent-bisexuality. Some of them are benevolent, but they all annoy her.
Subverted in Sluggy Freelance, where the images of Kiki and Jaya torment Bun-bun for abandoning them (they're ok, but Bun-bun doesn't know that). Telling them off in Card-Carrying Villain fashion cheers him up immensely.
Original Life features a creepy anthropomorphic cigarette (who is also smoking a cigarette) representing smoking withdrawal symptoms

Western Animation

Robin is taunted (and almost killed) by an illusion of Slade in the Teen Titans episode "Haunted."
The Powerpuff Girls once had to face an imaginary enemy that had powers in the real world; being unable to see or confront him, they had to concoct an imaginary friend of their own. Their combined brainstorming created a bunny lady scientist with a pretty dress and combat boots.
Subverted in Foster's Home for Imaginary Friends with Bloo. He's not a Complete Monster in any way...but when it comes to breaking Mac's cutie, he's there.