Imaginary beings often serve as projections of what a character is thinking, and can represent their desires, fears, or thoughts, so they always serve a key purpose. The question is what that purpose is, and if the intentions of the imaginary being match the protagonist.
This is most prevalent if the imaginary persona is serving as a sort of guide or mentor to keep the character in question grounded. Here's a look at a few in-universe fictional characters from both TV and film. Fair warning: There are a lot of spoilers for a few movies going forward, albeit many old ones.
Great: Mr. Snuffleupagus
To be fair, Mr. Snuffleupagas has been proven to be real, but for many years on Sesame Street, he was intended to be an imaginary friend. "Snuffy," as he is also called, is a friend of Big Bird, and since his inception in 1971, he was strictly imaginary, until he was made flesh shortly after 1985's Follow That Bird movie.
He was made real because the showrunners thought that depicting adult characters that constantly did not believe Big Bird would make real-life children think it was pointless to fill their parents in on critical issues. Snuffy is a loyal and down to earth friend, who just seems to have awful timing for coming and going.
Bad: Frank
Frank is a pretty creepy looking guy, to begin with, and in this case, the surface appearance matches the personality. In Donnie Darko, Frank constantly pops up to unnerve Donnie and instruct him to do unsavory things, like a twisted version of Harvey.
There's a lot to unpack with Frank and his purpose in the film, but surely he didn't have to be such a creep about it, especially in his unnerving bunny costume and disturbing voice.
Great: Bing Bong
Bing Bong was a pretty shifty character when he first appeared in Inside Out, and frankly, there was something just a little off about him. It almost felt like he was bound to turn evil at any second. But the writers certainly seemed to relish making their audience suffer for doubting Bing Bong's intentions.
As Riley's old and forgotten imaginary friend, Bing Bong has become obsolete and just wants to be relevant again, barely getting by in Riley's transforming personality and mindset. When he comes across Joy and Sadness, he helps them out, and when things go awry, Bing Bong makes the sacrifice and lets himself vanish from Riley's memory, in order to save Joy so his beloved Riley could be happy again.
Bad: Ivan
In The Machinist, Ivan distracts Trevor, a machinist suffering from insomnia, and the distraction causes a serious accident at the factory Trevor works at, ending with a coworker losing an arm.
This incident ends with Trevor getting the blame and Trevor going on a mad hunt to track down Ivan, all while the elusive Ivan mocking him at each step. Of course, it turns out to be a wild goose chase, as Ivan is a figment of Trevor's imagination, who hasn't slept properly in months.
Great: Wilson
There has never been a more iconic sporting good ever put to screen than the loveable Wilson from Castaway. Wilson helps keep the stranded Chuck alive by keeping Chuck's sanity intact for nearly half a decade of being lost at sea.
Named after the sporting goods manufacturer, Wilson is Chuck's best friend, and a piece of Chuck lives through him, seeing as his face is made of Chuck's bloody handprint. Tragically, Chuck loses Wilson while making his escape back to civilization, begging his companion's forgiveness in the ultimate realization of how lonely Chuck was.
Bad: Drop Dead Fred
Drop Dead Fred isn't the most well-known movie, and maybe that's for the better. The film is a complete tonal mess in how it approaches a woman going through trauma and re-creating her old imaginary friend to cope with it.
In an era filled with annoying movie characters (the 90s), it's impressive that Drop Dead Fred remains one of the most insufferably obnoxious characters, with no engaging or wholesome qualities to make up for it. In fact, most people will wish he would do as his name, and drop dead on the spot.
Great: Harry Morgan
Who knows how Dexter Morgan would have turned out if it wasn't for his foster father, Harry, who gave Dexter a code in order to kill only those who deserved it, and how to get away with it. In Dexter, Harry's voice is always there to guide the titular serial killer in carrying out justice rather than simple mass murder and serves as Dexter's conscience.
It makes it much easier to root for Dexter when he's out killing pedophiles, drug dealers, and fellow serial killers, rather than have Dexter be just another aimless killer, and that's thanks to Harry's code.
Bad: Tyler Durden
The nameless narrator of Fight Club comes across the flamboyant Tyler Durden, a soap salesman with a lot of sociopathic tendencies. Tyler seems fairly cool and somewhat admirable in his first few appearances, but things really get unraveled once he essentially goes from starting a fight club to making a terrorist movement.
Tyler Durden is the epitome of toxic masculinity, and he's nothing but a bad influence. Of course, he's merely a mental projection of the narrator, so really, he himself is the guilty party.
Great: Wilfred
Man's best friend is ... literally a man in a dog costume. In Wilfred, the titular character serves as the best friend of the majorly depressed and suicidal Ryan and is a projection of the actual dog of his next-door neighbor. No one else sees Wilfred as a man in a dog costume, but unlike most imaginary friends, they do see him (as he is a real dog).
Wilfred has a lot of vices, like drinking, smoking, and having a pottymouth, but he also counters his problematic nature with his advice. He is Ryan's most loyal friend and helps him out of the very dark and depressing hole Ryan found himself in, guiding him to self-acceptance and the re-discovery of happiness in his life.
Bad: Adolf Hitler
Who else could possibly take first place other than the führer himself, Adolf Hitler? This version of one of history's biggest monsters, as seen in Jojo Rabbit, is played for laughs, and yet, he still retains every last awful trace of Nazism in his character.
In the film, Hitler serves as Jojo's imaginary friend, an idealized version of the dictator as according to the naïve young boy's imagination. Of course, Hitler being Hitler, it's impossible to take away all of his negative aspects, even in the most idealized version, so Hitler loves to give Jojo terrible life advice and guidance.