The true form of a changeling is a bit paler than a typical human with slightly elongated limbs. Their eyes are colorless and ringed by black as pictured in the artwork above by Julia Zhuravleva. They have no body hair, but the hair on their head can be platinum, white, or silver.
Obviously, a changeling's appearance can change at any moment, but they do have a true form. Their true skin is usually grey or pale white, and their facial features are blank slates to an unnerving degree. They're typically thin and gaunt, and their hair is usually silvery with an occasional green or pink tint.
While changelings can look like anyone, they do have a true form. Their natural look can be scary to some due to their lack of detail and distinctive features. Their skin tone is always pale, either white or light gray, and they tend to have slender bodies with limbs slightly longer in proportion to other humanoids.
In their natural form, changelings stand between five and six feet tall, with pale gray skin, thin, fair hair, and disproportionately long limbs compared to humans.
Most changelings don't know they're changelings.
Though they may always feel somewhat different or removed from human life, most changelings are too young to understand who they truly are.
The new child—the changeling—is characterised by unresponsiveness, resistance to physical affection, obstreperousness, inability to express emotion, and unexplained crying and physical changes such as rigidity and deformity. Some are unable to speak.
Most often it was thought that fairies exchanged the children. In rare cases, the very elderly of the fairy people would be exchanged in the place of a human baby, so that the old fairy could live in comfort, being coddled by its human parents.
Changeling blood is green, unlike most other animals. Changelings, despite their thinly and seemingly fragile frame, are capable of surviving otherwise fatal injuries thanks to their magical abilities.
Unlike doppelgangers, which are entirely genderless, a changeling does have a default gender that manifests in adolescence, but each changeling can adapt his or her form to be of either gender, hermaphroditic (both genders simultaneously), or entirely genderless.
Changeling Traits
Age: Changelings reach adulthood at the age of 16. Their fey heritage allows them to live much longer than humans, up to 200 years.
Changelings don't inherently recognize each other. If they don't want to be recognized then when they do something suspicious it would be an insight check against the changelings deception, which the changeling has advantage on. A changeling could have expertise in insight from divergent persona.
Changelings are a species of elf-like demons with pale skin and pointy ears. They are connected to the forests in which they dwell. Changelings are known to place their young in the homes of mortals so they can grow up in safety.
Changelings cannot alter themselves enough to change physical sizes (i.e, changing from Medium to Small, etc). It is worth noting that while a changeling could technically shift into something like an aarakocra, their wings would be non-functional.
The short version is that a Changeling can shapeshift to a form that can produce sperm to impregnate someone, and can shapeshift to a form with a uterus and eggs to get pregnant, but a pregnant Changeling cannot shapechange to a form without a uterus without ending the pregnancy.
Changelings are thought to be an uncommon race—though their true demographics remain unknown, as may live their entire lives masking as other sentient races. Many places in the world fear changelings for their unique ability to assume the form, or 'profile', of nearly any person, indefinitely.
Appearance. In their true form, or their form "at rest", changelings have pale skin, colorless eyes that can be completely white or completely black, and white or silver-white hair.
Changelings have an interesting relationship with gender as a concept, which is probably part of the reason a lot of the art doesn't find it necessary to distinguish the males with facial hair. As they're changelings though, they can have any kind of hair they want.
As an action, you can change your appearance and your voice. You determine the specifics of the changes, including your coloration, hair length, and sex. You can also adjust your height and weight, but not so much that your size changes.
Vocal Replication: All Changelings can mimic any sound they hears using their voice. Voice Manipulation: All Changelings can freely manipulate their voice, allowing them to control their voices to imitate sounds of creatures such as animal noises and explosions or increase or decrease the tone of their voice.
Color. The color of human blood ranges from bright red when oxygenated to a darker red when deoxygenated.
Some types of octopus, squid, and crustaceans have blue blood. Their blood contains a high concentration of copper. When copper mixes with oxygen, it gives their blood its blue color. Blue and red are not the only possible colors of blood — some animals bleed green.
Babies are most at risk, although the Sidhe fear nothing more than iron and fire, so a pair of scissors or tongs may be left near to the crib, or the crib left close to a fireplace, and draping a garment belonging to the father of the child over the baby as it sleeps is also said to be a preventative.
Other times, Changelings don't speak at all, or show a lack of intelligence, and tend to become violent, screaming and biting at their parents. Another sign of a Changeling child was unusual behavior such as jumping, dancing, and spinning.
By RAW changelings revert when they DIE but not when they are unconscious (it would be hard to maintain a long term masquerade if you revert when you sleep!). I bet suggested that a piece of a Changeling — such as cut hair — WILL revert, but that this is slow and may take up to 24 hours.