Empathic Shapeshifter

Sometimes The Mind Is a Plaything of The Body when you've transformed into something else. Well, sometimes the body is the plaything of the mind. Or the world.

There are some characters whose physical form is involuntarily dictated by their environment, or the people around them. Sometimes this is useful; perhaps the character can instantly adapt to new environments. More often, this can be a pretty serious case of Blessed With Suck: you can transform, but you'll only turn into whatever other people think of you. Which wouldn't be a bad thing if you're hanging out with your close friends, but even Cthulhu shudders to think about what will happen should you be, say, Paris Hilton.

Empathic shapeshifters very often have some form of Adaptive Ability as well.

See also Clap Your Hands If You Believe and Something for Everyone.

Can be related to A Form You Are Comfortable With, if it's implied that people merely perceive the shape they want to (to understand the difference between these tropes, ask Irwin Schroedinger).

Anime and Manga

 * In Osamu Tezuka's Phoenix: A Tale Of The Future, the Moopies are amorphous aliens who will voluntarily become anything a person desires. Earth society condemns interacting with them, because it is of course tremendously addictive. The main character is in love with a Moopie who has turned into his idea of the perfect woman.
 * In Revolutionary Girl Utena, it's heavily implied in the Black Rose Arc that.
 * in Rah Xephon will take the form of the whomever  cares about the most.

Comic Books

 * X Men
 * Meggan, from the spinoff Excalibur. She is the Trope Namer as she was explicitly described as a Empathic Shapeshifter and, indeed, she would look like what other people thought she should. This caused her quite a lot of angst as a kid, though she fortunately had other powers to back it up and had the ability to adapt to any environment bundled in. However, she managed to move past it as of the Captain Britain and MI:13 series.
 * Darwin from the X Men would gain new powers to protect himself from threats, often shape shifting in the process.
 * Lifeguard, also of the X-Men, teleported situationally, developing powers that would be useful to save any endangered lives around her.
 * Earth X suggested that all of Marvel Comics' supers were evolving toward this state. Loss of Identity soon follows.
 * Trauma of Marvel's The Initiative can touch the mind of his target and turn into their greatest fear -- complete with that fear object's abilities.
 * One of the stories in Demo has a girl with this kind of power, but it only applies to romantic intent. She's searching for someone who will see her the way she really is. She finds a woman who seems to be able to do just that and falls in love with her based on that fact alone, only to find out that the woman is already seeing someone (and hence implicitly didn't care about making her look like the kind of person she already had).
 * Wild Cards
 * Succubus, who becomes the ideal lover of whoever she's near, and has since childhood. Her parents actually took advantage of this and pimped her out as a prostitute (her clients were mostly unaware of her real age since she looked older even in her default form).
 * All Jokers have a form of empathic shapeshifting, except it's one-shot only (during initial infection), self-imposed, unconscious and uncontrollable, and ends in permanent Shapeshifter Mode Lock. According to Dr. Tachyon, the Wild Card Virus takes a random-- or maybe not so random-- image from the subconscious, and amplifies it to telekinetically influence how the virus is rebuilding the body. This explains why some jokers have animal or mythological forms, and why some Aces have overly specific powers.

Film

 * Prot in K-PAX claims that he appears human on Earth because it is the most energy efficient shape he could assume here. He uses the analogy of "Why is a soap bubble round?" and says that on K-Pax he appears K-Paxian.

Literature

 * Mix a little of this with a whole lot of Nightmare Fuel and you've got the insane children's book A Bad Case of Stripes.
 * The Boggart in Harry Potter.
 * In Clive Barker's Imajica, one character is a "mystif" who takes on the form matching the unconscious sexual desires of whoever looks at him/her/it. The character says that many people, when shown what they actually desire instead of what they think they desire, end up feeling horrified instead of attracted.
 * The Warlock In Spite Of Himself series has Witch-moss, an alien fungus that adapts its shape in response to Telepathy-- even unconscious telepathy, explaining why there are and many other mythical monsters on the planet Gramarye.
 * In James Blish's The Duplicated Man, the title duplicates are formed by a machine that is controlled telepathically by its operators. The operators are displeased to discover that all the duplicates come out wrong because each of the operators has his own imperfect, thoroughly subjective ideas of what the original guy is like, both in his personality and his appearance.

Live Action TV

 * Skinfred, a bizarre one-off character from Fraggle Rock, was completely at the mercy of other people's opinions of him. It's really amazing how upbeat he was come to think of it.
 * Red Dwarf
 * A one-off character on looked like whoever the viewer was most attracted to -- The very personification of beauty is in the eye of the beholder.
 * The Psirens from a later series could take the form of their chosen victim's greatest sexual fantasy, complete with voice and mannerisms, etc, only they did it to get brains to snack on.
 * The Monster of the Week in the The X Files episode "X-Cops" kills people using the form of whatever they're mortally afraid of. Forms it's assumed have included a werewolf, "the Waspman", Freddy Krueger, a crackhead, and the hanta virus.
 * A one shot patient in House had a bizarre type of amnesia, where he would automatically adopt the mannerisms and personality of whoever the dominant person in the room was.
 * An episode of Star Trek the Next Generation featured a woman who was to be used to seal a peace treaty: she would marry the leader of a planet and end a centuries long war. The problem is, while she remains physically the same, her personality changes to whatever the men around her find irresistibly attractive, which has the added effect of her being irresistibly attracted to every man she meets, and whoever she first has sex with, she'll Shapeshifter Mode Lock into that person's ideal personality permanently. So the problem is, managing a way to safely transport her to her destination without her losing her virginity.

Role Playing Games

 * The Tulpas in Over the Edge are formed by what their opposite expects them to be.

Video Games

 * City of Heroes': This, combined with Power Perversion Potential, can be found in...certain RP cliques on one of servers. ...*shuhdder*
 * Pokémon:
 * Eevee: every game describes it as having an unstable genetic structure, and it evolves into one of several creatures depending on environmental stimulus (or hitting it with a magical stone). So far, we have evolutions for seven habitats: hot/fire, aquatic, electrical, nocturnal, diurnal/friendship(?), forest and cold/snowy. That said, once it evolves it's permanent, and since there are reliable ways for Trainers to force/invoke each individual evolution...yeah.
 * Ditto, however, can transform into anything it sees regardless of size. In the games, it can only transform into other Pokémon, but in the anime it has been shown to mimic things like objects.
 * The Dog in Secret of Evermore takes a form to reflect whatever region of Evermore the Boy is exploring at the time.

Web Original

 * Whateley Universe
 * Most mutants actually change their appearance (involuntarily and unconsciously) to match their ideal self-image. Again, usually comes with Shapeshifter Mode Lock.
 * Fling shapeshifts according to other people's sexual fantasies, of any sex or sexuality.
 * Another secondary character shapeshifts into whomever she's looking at.
 * The SCP Foundation has two of these. The first is a being that becomes "better" than anyone observing it. It will take on a shape that is more attractive, stronger, and smarter, with the particulars determined by the mindset of the observer. Even when there aren't any people around, if there is a camera monitoring it, it becomes a higher quality video camera. The second one is an object that transforms into something that will have no current use for the person holding it. For example, if you are cold, it would become an ice cube. If you are trying to put out a fire, it would become something flammable.

Western Animation

 * Ex-Super Soldier Shane "Goose" Gooseman of Adventures of the Galaxy Rangers shapeshifts automatically in response to any threat (forming reflective armor against Frickin Laser Beams, for instance), but if his Series Five implant goes out of whack, he loses control of his power and becomes a danger to himself and others.

Real Life

 * The chameleon. They change colors based on features of the environment like temperature. Despite the common notion, their colour change is not so much based colour of the environment. A naturally green chameleon will not change to pink or brown if you pick it up or move it to a piece of tree bark. Apart from specific patterns for things like mating, most of the colour changes are darkening, brightening or paling.