Kwamis are sprite-like, "abstract" creatures that give power to people with Miraculouses to transform into animal-themed super beings.
As explained by Master Wang Fu, Wayzz, and Tikki in the second issue of Miraculous Adventures, kwamis are formed whenever a new abstract idea or emotion comes into existence in the universe, such as creation, love, beauty, and mathematics. When these emotions came into being, kwamis were formed at different times, such as Tikki, the kwami of creation, who was formed at the beginning of the universe and thus was the first kwami (and being) to exist. However, the kwamis couldn't be spotted by regular tangible creatures like humans. While they were able to journey across the universe, kwamis weren't sensed by humans and were unable to interact with them.
The issue was solved thousands of years ago, when a mage figured out how to create magical jewels, Miraculouses, in order for the kwamis to be able to communicate with and assist people. From that point on, kwamis helped the wearers of their Miraculouses achieve good in the world, although there were instances where they became stuck with villainous holders and forced to help them with their diabolical schemes.
Generally, kwamis are small, being around 10 centimeters in height, with large heads and tiny bodies. They have arms and legs, but they lack fingers and toes. The animal theme that their Miraculous has affects their appearances, such as Tikki being red with black spots and antennae like a ladybug and Plagg being black with green eyes, cat-like ears, and a cat-like tail.
When drinking certain potions made from the Miraculous spellbook their appearances will change. If the aqua mode potion is used, the kwami will gain fins and a tail. Or when it comes to "ice mode", the kwamis gain crystals on their bodies
Normally, kwamis can fly and levitate. They can also become intangible to pass through solid objects, like locker doors. Even though they are small, kwamis appear to have enough strength to hold things that are larger and/or heavier than themselves. They can live for thousands of years—possibly forever—having been around since before the creation of the Miraculouses. Despite their ages and powers, kwamis can get sick, as seen in the episode "Princess Fragrance" when Tikki becomes sick after helping Marinette defeat a villain in pouring rain. Wang Fu is able to assist in their recovery and heal them. In "The Collector", Plagg notes that kwamis aren't able to be filmed or photographed, not showing up in photos and videos to prevent others from discovering their existence.
Kwamis inhabit pieces of Miraculous jewelry, like earrings or a ring, in order to give the person wearing them the ability to transform into superheroes and gain a specific power based of the abstract idea or emotion that made the kwami come into existence, such as Plagg granting the power of destruction and Tikki the power of creation. Kwamis are aware of the actions of the people who possess their Miraculous when transformed. However, they cannot communicate with their holders when they are transformed. For example, Plagg laughs at how Adrien said rude things to Ladybug when he was possessed by Dark Cupid's arrow as Cat Noir in the episode of the same name. At the same time, the extent they can be aware what is happening when their Miraculous is used is uncertain; in "Ladybug & Cat Noir", Tikki and Plagg appear to be unaware whether the akuma was captured or not.
The transformation can last indefinitely unless the Miraculous' special power (e.g. Ladybug's Lucky Charm or Cat Noir's Cataclysm) is used, at which point the wearer has about five minutes until they revert to their normal form and the kwami leaves the Miraculous. To regain power, to strengthen themselves, and to transform their wearer again, kwamis need to refuel by eating food. Some kwamis have a preference for specific foods: for instance, Plagg's favorite food is Camembert cheese, but he is willing to eat other types of food if Camembert isn't available for him.
When they are linked to a Miraculous and their Miraculous is not in use, a kwami can roam freely outside of the Miraculous. As revealed in "Ladybug and Cat Noir", when the current holder disposes of their Miraculous, the associated kwami disappears and goes back into the dormant Miraculous. When the Miraculous gets put on again, whether by the same user or a new one, the kwami comes out of the Miraculous. If the Miraculous comes off by accident, though, like when Adrien's ring falls off in "Lady Wifi", the kwami is not pulled back into the Miraculous. The kwami will only disappear if the holder wants to get rid of the Miraculous.
Kwamis also perform a ritual for their Miraculous holders where they create a kwagatama inside their bodies from the hairs of their past and current holders. They then present the charm to their current bearer for a special occasion, such as a birthday.
Kwamis can communicate with each other once in every “cycle” on their birthday, if they gather in the Miraculous jewelry box at a specific time and the more kwamis there are the stronger the connection.
Potions from the Miraculous Spellbook are able to alter a kwamis appearance and form by drinking the potion directly or consuming something that contains it. Besides modifying a kwami's appearance and form, potions also help the wielder get new abilities and appearances (in their superhero form) depending on the potion the kwami consumed.
The known power ups are:
Aqua - The kwami's legs are replaced with a fish tail, and their hands are replaced with flippers. It allows the kwami to grant their holders underwater abilities, and change their appearance with additional flippers on their feet. The potion's color is green.
Ice (name unconfirmed) - The kwami grows three ice shards somewhere on its body (which vary between kwamis), and in Tikki's case, her circular spot turns into a hexagon. It allows the kwami to grant their holders ice related power. The potion's color is light blue.
It is unknown if kwamis can be akumatized, but Season 2 will have the answers, as revealed at the Miraculous Ladybug panel at SDCC 2016.
According to Jeremy Zag, Season 2 will include an episode revealing the origins of the kwamis.
The New York Comic Con 2016 panel additionally noted that there will be webisodes that focus on the kwamis, notably Tikki and Plagg, and that there may be more than one episode in new seasons that centers on them.
The official Miraculous Ladybug Twitter account noted that while more kwamis will be seen in Season 2, it won't be all of them.
According to Thomas Astruc during a Miraculous panel at Ludicomix 2018, the kwamis are inspired by the Chinese zodiac signs because he wants the world to get to know the elements of the oriental culture.
A teaser trailer released on August 31, 2017, revealed that some of these new kwamis are based on the animals of the Chinese Zodiac, though a kwami based on the rabbit was not included or confirmed.
In "Syren", it's revealed that kwamis must never know the spells that are inside the book, as a precaution should a kwami fall into the wrong hands.
Kwamis feel a sense of familiarity with the book when they see it. Plagg does not know exactly what it is when he first sees it but knows he's seen it before, and Tikki quickly recognizes it, stating that it must be brought to Wang Fu immediately.
Even though kwamis are familiar with the book, they don't know how to read it, as revealed by Nooroo in "Syren".
In "Sandboy", it's revealed that kwamis can communicate with their kind once in every “cycle”, if they gather in the Miracle box at a specific time and this cycle is referred to a kwami's birthday.
Also according to Tikki a kwami's cycle or birthday is equivalent to several hundreds of human years.
Also in "Sandboy", according to Tikki kwamis aren't allowed to leave their miraculous wielder's side in case someone becomes akumatized.
In the case with Trixx and Alya, since she is temporary Miraculous holder and has to return the Fox Miraculous at the end of a mission.