You Are Who You Eat

"Tell me what you eat, and I'll tell you who you are."

- Jean Anthelme Brillat-Savarin, gastronomy expert (quote taken from a particular cooking show)

No one takes the saying "you are what you eat" literally; it's not as though being a vegetarian will make you a Plant Person, or eating pure beef will make you a Minotaur. Some non-human critters didn't get the memo though, because for them eating people means being people. For some supernatural, alien, or stranger creatures to pretend to be human at all requires that they make a periodic consumption of Human Resources. Or simply put: Ghoulie has to eat people to look like one.

For the non-human, this will allow them to shape shift into a human form, potentially that of the eaten, though it may only allow them to become a human version of themselves. The non-human may use non-cannibalistic methods as well, for example a fae creature might steal the shadow of a human to maintain their Glamour, a Mutant might graft skin (or new appendages from healthy humans to avoid Power Degeneration, a robot or alien may Kill and Replace, and a vampire may have to drink blood to avoid turning into a hideous monster. It's worth noting that perfectly normal humans may be able to do this through a spell, ritual, or if they have the Cannibalism Superpower.

Depending on what the non-human takes is how severely the victim is injured or killed. Some may be able to make a complete recovery, others will be scarred or crippled for life, and some unlucky souls will end up an Empty Shell. Needless to say, this makes being nice pretty difficult.

Most non-humans do this in order to maintain a Masquerade, and pose as human. Especially Tragic Monsters may be doing this because they want to Become a Real Boy. Especially monstrous ones will enjoy Showing Off the New Body. If the non-human doesn't eat people, whether out of niceness or inaccessibility, they will revert to their Shapeshifter Default Form. Things may go From Bad to Worse from there though if that entails turning into an unthinking, horrific monstrosity that can never fit in among humans. And of course things will Go Horribly Wrong if their cannibalism is also caused by a Horror Hunger, and deprivation has the nasty side effects of making them dangerously hungry. Subtrope of Face Stealer

Anime and Manga

 * Buu of Dragonball Z possessed an absorption attack, which allowed him to take on aspects on an enemy's powers, intelligence, and appearance.
 * The heroes assumed he grew stronger with each absorption and so freed their comrades from Buu to revert him to a less powerful state. Unfortunately, they went too far and freed Fat Buu (who Evil Buu had absorbed), causing him to revert to his original state, which was even more psychotic than the others.
 * Youma in Claymore can take the appearance of those they eat, and this also gives them the ability to draw from their victims' memories to fool their families (and eventually eat them too).

Comic Books

 * Everyman of the DC Universe actually only needs to eat a tiny bit of whoever he wants to turn into. But it turns out he really likes the taste of human flesh.
 * The Saurians in Sigil (and the one who appears in Negation) take on beneficial attributes of whatever creatures they eat, including physical and mental traits and even information. Having long ago become the apex predators of their homeworlds, this power was forgotten and rediscovered when they went to war with humans and decided not to waste the corpses of their fallen foes.

Film

 * The vampires in Daybreakers have to drink human blood or else they transform into subhuman monsters. Expanding on this, drinking vampire blood (even their own) causes the mutation to accelerate.
 * In Men in Black, the alien roach skins a farmer near its crash site and wears him to pass as human. As the film wore on the skin decayed more and more, making the impersonation all the creepier. They call it an "Edgar-Suit".
 * Wesker in the film Resident Evil: Afterlife needed to feed on uninfected humans to keep from going the way of the boss monsters in the last three films.
 * The Thing is based on this trope.
 * Phantoms (1998) featured underground creatures that absorbed the memories of the humans they ate. The film claims flatworms can do the same thing but that was disproven long before the script was written.

Literature

 * In Harry Potter, polyjuice potion requires as an ingredient a few hairs from the person being impersonated. This is a particularly nightmarish example because the person has to still be alive when the hair is taken, meaning anyone attempting long term infiltration / impersonation has to keep the original alive and captive, which is what happens to.
 * Something less creepy: "If the saying 'You are what you eat' is really true, then Lord Voldemort is a unicorn."
 * In the Doctor Who New Adventures novel Human Nature (which was later adapted for the TV series, but without this aspect), one of the members of the Family is a shape-shifter who can imitate any animal he's eaten part of, including humans. If he does it while they're alive, he can also gain their memories.
 * A nightmarish version occurs in one of the novels in Andrey Livadny's The History of the Galaxy books, where a human ship arrives on a habitable world that does not appear to have any animal life. Then the humans get picked off one by one by strange shapeshifting creatures. In the end, all humans have been consumed, but the shapeshifters also absorb their memories along with their DNA and begin to think of themselves as human, not understanding why they are suddenly amorphous blobs able to form into anything they wish. Another novel has a scout ship re-discovering the planet only to realize the shapeshifters are intelligent. The captain of the scout ship doesn't care and wants to exterminate them in order to claim the planet and retire on the fortune (habitable worlds are extremely valuable). The protagonist manages to get rid of the captain and leave, wiping the location of the world from the navigation computer, so the creatures can live in peace.
 * Yet another novel reveals that the shapeshifters are the result of an ancient experiment to create immortality. It worked. The resulting creatures went out of control and began consuming all the surrounding fauna, eventually leaving the half-dozen shapeshifters the only non-plant organisms on the planet. The same experiment on another world had slightly better results.
 * In Mistborn, the kandra are creatures who can take on the appearance of a person after consuming their bones. They do this for humans as a service for a very specific kind of payment. They are literally contractually obligated not to kill humans, though. Their employers provide the corpse.
 * Particularly skilled kandra can assume someone's form without having eaten them. They still need a skeleton, though, since they're basically Blob Monsters in their true forms and don't have any rigid bones of their own.
 * This is in The Book Of The Short Sun;
 * In The Throne Of Bones, ghouls' tendency to assume the appearance and identities of those they devour—even to the point of forgetting that they're ghouls—is pivotal to most of the stories.

Live Action TV

 * One demon in season 1 of Buffy had to eat the brain of a human in order to maintain his disguise.
 * In the Doctor Who episode "Smith and Jones" there's a plasmavore who already looks human, but has to drink human blood in order to scan as human.
 * The Wraith of Stargate Atlantis look humanoid, but are actually technically insects. Thousands of years of feeding on human energy/life force introduced trace amounts of human DNA into their gene pool, which caused the many-legged, foot-long insects to evolve into the sentient, two legged, two armed, human-sized, human shaped Wraith of the present.
 * In the Fear Itself episode "The Eater", the eponymous Serial Killer Claude Mellor consumes parts of his victims in order to assume their forms. It works.
 * Jasmine, on Angel, who had to eat people in order to heal herself and maintain her human appearance.
 * A two-part episode on Sliders involved the logistics behind moving a select portion of the population before their Earth is destroyed. It turns out that the man in charge has to ingest brain tissue from compatible donors due to treat a fungal brain infection. Injecting the tissue causes him to temporarily morph into the person while the donor goes into a coma. The list of people he handpicks to move on to the new world are all compatible; he intends to use them all to stay alive.

Music

 * German Band Knorkator has the song "Ick wer zun Schwein" (I turn into a pig). It's about a man who ate a really large steak all by himself, which causes his DNA to be rewritten and turn him into a pig.

Tabletop Games

 * Call of Cthulhu. The Consume Likeness spell allows a sorcerer to take the form of a dead person by eating their body.
 * In Warhammer 40,000, the Kroot and Tyranids evolve primarily by eating other beings and absorbing their genetic traits. In fact, the reason Kroot are able to build spaceships is because they ate a bunch of Orks and absorbed the Mekboyz' genetic memory. Kroot also have a taboo against eating Tyranids to avoid creating some sort of horrendous feedback loop. In-Universe, to avoid accidentally absorbing the part that plugs those bugs into Hivemind.
 * Lunar Exalted from Exalted can take the form of any creature whose heart's blood they've tasted.
 * Skin-Changers from GURPS Monster Hunters are Embodied Spirits who can shapeshift into any animal (including humans) by wearing their skin. Unlike the Men in Black example, Skin-Changers are clever enough to preserve the skins, and save them in jars when they're not being worn.

Theater

 * On the Verge, or the Geography of Learning has an unusual example Played for Laughs: A cannibal gets confused about his own identity thanks to ingested memories.

Video Games

 * Kirby is able to assimilate the special abilities of many of his enemies by swallowing them. Fan works often exaggerate this to Kirby assuming the characteristics of anyone and/or anything he eats—notably, this is the Running Gag of Brawl in the Family.
 * Vampires in The Elder Scrolls: Oblivion have this in effect, not feeding regularly (a good thing!) will make you scare off anyone you come near (a bad thing, no quests!). Interestingly, this also gives you access to lots more vampire powers.
 * In Prototype, Alex Mercer's disguise ability works like this. When he consumes the individual, he uses their genetic code to reproduce their appearance and voice while their memories are directly absorbed.

Web Comics

 * In Sluggy Freelance, zombies have to eat the brains of the living or they'll lose their own intelligence.
 * In Demon Eater, this is the main way of growing in the demon world. Demons must eat demons, or be eaten themselves.
 * Plastic Dudesmen speculates what happened to the Hive Fleet that devoured the Squats. Since their toughness is inseparable from innate addiction to beer… somewhere, there's a swarm of very beardy bugs that won't be sober enough to drive a Hive Ship any time soon.

Western Animation
"Mandy: If you are what you eat, I could be you by morning."
 * In The Grim Adventures of Billy & Mandy Trapped in TV Land episode, Pinocchio wants to be a real boy by eating human flesh, specifically Billy's.

"Dib: I suppose you have a heart? Zim: Six of them. Dib: Intestines? Zim: Large and small. Dib: Spleen? Zim: In three different colors."
 * In Invader Zim, Dib tells Zim that he knows he's an alien because humans don't have a squeedlyspooch, and Zim doesn't have any normal human organs. Zim's solutions is to stalk his classmates, steal their organs, and stuff them inside himself.


 * At one point the nurse tells Zim he's extra human for having multiple organs, while Dib almost gets hauled off by the MIB for missing some.