Kwami are sprite-like creatures that give power to people with Miraculouses to transform into animal-themed super beings.
Generally, kwami are small with large heads and tiny bodies. Whatever animal theme that their Miraculous has, affects their appearances, like 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.
Normally, kwami can fly and levitate. They can also become intangible to pass through solid objects, like locker doors. Even though they are small, kwami appear to have enough strength to hold things that are large and/or heavy. They live for thousands of years — possibly forever — having been around since the creation of the Miraculouses.
Kwami 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, such as Plagg granting the power of destruction and Tikki the power of creation. 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 transform their wearer again, kwami need to refuel by eating food. Some kwami have a preference for specific foods: for instance, Plagg will only eat Camembert cheese.
According to Thomas Astruc, the name of every kwami has to be a short name with a double letter in it.
In "Timebreaker", Tikki is able to sense her own power in the Ladybug from the future.
Similarly, Wayzz is able to sense the power of Nooroo and the Moth Miraculous in "Ladybug & Cat Noir".
Tikki reveals that she is more than 5,000 years old in "The Pharaoh", suggesting that kwami are immortal.
Kwami 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.
An example of this is when Plagg laughed at how Adrien said all the rude things to Ladybug when he was possessed by Dark Cupid's arrow as Cat Noir in the episode of the same name.
In "Ladybug & Cat Noir", Tikki and Plagg appear to be unaware whether the akuma was captured or not, so it is actually unsure to what extent they can be aware what is happening when their Miraculous is used.
Kwami are like Aladdin's genie, having to obey their holders' desires.
Kwami can get sick, as seen in the episode "Princess Fragrance", when Tikki becomes sick after helping Marinette defeat a villain in pouring rain.
Master Fu is able to heal them.
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, 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.
It is unknown if kwami 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 kwami.
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.