Chao are small, cute creatures that appear in the Sonic the Hedgehog franchise. They made their first appearance in Sonic Adventure on the Dreamcast, serving as both a plot device and as a form of virtual pet for the player. They have made numerous appearances in other Sonic games since then, usually as background characters.
Chao are small creatures with soft, jelly-like bodies. By default, they appear to be light blue with yellow patches on top of their heads, the ends of their arms and feet and on their rumps. They also have small pink wings on their backs and an emotion ball hovering just above their heads that changes shape based on the Chao's mood.
Chao can be quite complex beings. They typically behave much like human babies and make gibberish vocalisations as they interact with one another and other creatures. They are very emotional and respond to various stimuli depending on how they are treated. For example, Chao enjoy being petted and love being in the company of people who treat them well. Likewise, they fear those who bully and hurt them. The disposition of a Chao depends greatly on the nature of the person who raises them and over time the Chao will change in shape and in manner based on how it is raised.
One of the most interesting aspects of the Chao is their ability to absorb the DNA of other animals. When a Chao makes physical contact with certain animals, parts of its body will change shape and it will gain the attributes of said animal. For example, touching a bird will increase its flying ability, but touching a heavy land-based animal like a gorilla will increase its strength, but reduce its flight stats.
Chao change their form depending on how they are cared for. There are three main classes of Chao: Neutral, Hero, and Dark. Each class possesses different physical traits that change depending on the Chao's ability stats.
All Chao are born neutral and begin life with light blue bodies and yellow patches on their limbs, heads and tail-ends. Maintaining their neutral status as they develop requires that they be given equal amounts of attention from heroic or villainous characters. Their colours and shape alter depending on how their stats are managed, which change when they are given small animals or different types of Chaos Drives (devices acquired from robots in Sonic Adventure 2). Red Chaos Drives indicate the Power stat, green for Run, purple for Flying and yellow for Swimming. If all of these stats are kept equal by the time a Chao grows to its adult phase, its appearance will be mostly unaltered and will have only increased slightly in size. Its yellow highlights may also turn green.
Power-types develop a red or orange colour and their limbs become thicker and stronger. Their heads also grow small horn-like protrusions.
Speed-types usually turn a blueish-green colour, though they can also turn green or even purple depending on how high their other stats are. Speed-type Chao also grow spines on their heads that make them resemble Hedgehogs.
Flight-type Chao will turn various shades of pink and their wings will grow. Two long striped horns will also grow out of the back of their heads that droop back and down, giving them a somewhat clownish appearance.
Swim-type Chao develop very aquatic qualities when they reach adulthood. Their bodies turn green and yellow and their limbs alter into fins. Their wings also curve upwards and look more like fins than wings.
Infant Chao will grow into Hero-class Chao if they are raised kindly by Hero characters or abused by Dark characters. Their bodies turn white with blue highlights, their eyes turn black with white pupils and their floating baubles also turn into blue rings resembling halos, giving them an angelic appearance.
Standard Hero Chao have two small nubs atop their heads which will change shape depending on how their stats are altered following their transition to adulthood.
Speed-types have a single striped antenna that ends in a point. If their power stat is significantly raised, the antenna will develop a bulbous tip. If their flight stat is raised, the antenna will turn into a purple-striped fin. With the swim stat's increase, the antenna splits into a tree like fronds. If the run stat is raised, it splits in three.
Swim-type Hero Chao develop aquatic-green markings on their torsos, arms and feet and their antennae curve into a leaf shape. The antennae elongate at different angles depending on which stat is raised the most, though when the power stat is raised the antennae become shaped like rabbit ears.
Power-type Hero Chao develop reddish orange markings and have three fat antennae atop their heads that alter shape depending on further stat alteration. They are also noticably more chubby than other types.
Flight-types develop purple markings on their limbs and their antennae curve back and thin out, resembling feathers. Their wings are visibly larger and more streamlined.
Infant Chao will grow into Dark Chao if they are raised by Dark characters (such as Shadow) or abused by Hero characters (such as Sonic). Their bodies turn black with red highlights, their eyes turn blue with white pupils and their floating baubles become spikey. Their tails grow longer and pointed - like a cartoon devil - and the Chao's antenna grows crooked. Their wings also become red and webbed.
The highlights of Dark Chao change colour depending on which of their ability stats are raised. Normal Dark Chao don't change much after reaching adulthood, though having their swim stat raised causes the antenna to droop down the back of the head.
Power-types become stockier, with bulkier arms and chubbier cheeks. Their highlights also become striped and their antennae protrude upwards, though still crooked.
Speed-type Dark Chao, like the Neutral class, develop quills on their heads. These quills are more crooked and curve upward; they also have coloured stripes running along them, making them resemble Shadow the Hedgehog.
Flying Dark Chao develop horns resembling a jester's hat, much like the Neutral flying Chao. The difference is that their coloured markings encompass their whole heads apart from the face, completing the jester hat look. Flying types with their strength stat raised look a lot more devilish since their horns are tinted red and curve upwards and inwards.
Swim-types develop luminescent markings on the belly and muzzle. They also grow jagged horn-like extensions from their heads which share the same coloured highlights.
Creating a Chaos Chao is a long process. In order for a Chao to become a Chaos Chao, it needs to have reincarnated at least twice. How the player raises the Chao in its previous lives has absolutely no impact on whether or not a Chao can become a Chaos Chao. After a Chao transforms for the second time, the player can give it at least one of every small animal in the game. The player can still feed it fruit, and should do so to keep its Chao happy. If all of the requirements are met, when the Chao evolves, it should come out of the cocoon as a Chaos Chao.
Chaos Chao have many unique traits. They are immortal, never reincarnating or dying. Chaos Drives and small animals don't affect the appearance of Chaos Chao, though they can still come in different colors. Chaos Chao also cannot mate. Though their behavior remains the same before they transformed, they can no longer change their facial expressions with the exception of falling asleep and having their nap interrupted, and are still able to blink.
Neutral Chao that evolve into Chaos Chao become Light Chaos to resemble Chaos (From Sonic Adventure), Dark Chao that evolve into Chaos Chao become Devil Chaos, and Hero Chao that evolve into Chaos Chao become Angel Chaos. Chaos Chao have an ominous, organ-like jingle when they transform.