Miraculouses are magical jewels. Worn as jewelry, they have the ability to change their wearers, with the help of kwamis, into animal-themed, super-powered beings.
As explained by Master Fu, Wayzz, and Tikki in the comic story "The Trash Krakken Part 1", kwamis have existed since the beginning of time. Being "abstract" creatures, each kwami formed whenever a new idea, concept, or emotion was formed in the universe, like creation, mathematics, and love. However, tangible creatures like humans could not see or sense kwamis, so the kwamis could only roam the universe without communicating with them.
Thousands of years ago, a mage successfully helped kwamis interact with humans by creating magic jewels, which were called Miraculouses. Because of the kwamis inhabiting them, the Miraculouses bestowed amazing power to their wearers. There are two Miraculouses that are more powerful than the rest: the Ladybug Miraculous, having the power of creation from the first kwami in existence, Tikki; and the Cat Miraculous, having the power of destruction from Plagg. In "Ladybug & Cat Noir (Origins - Part 1)", Nooroo notes that legend says when the Ladybug Miraculous and Cat Miraculous are worn together, their user will gain god-like powers. At some point in time, a lot of information about the Miraculouses and their powers were written down in code into a book, with very few knowing how to decipher the code. A jewelry box was also created at an unknown time to store the inactive Miraculouses.
According to Nooroo, the Miraculouses, meant for good purposes, have been used by heroes throughout history. However, in the wrong hands, Miraculouses may be used for evil.
Usually, a Miraculous is assigned to an appropriate user with good intentions by the Great Guardian, a human tasked with watching over all of the Miraculouses and keeping them in a safe place when not being used. While there is a chance of giving a Miraculous to a wrong choice, as Master Fu has done once with an unknown individual, the system prevents a lot of poor options for Miraculous holders from getting Miraculouses easily. The Great Guardian takes back and stores the Miraculous when it is discarded or returned to him. Unfortunately, there have been cases where Miraculouses are lost, even potentially forever, and the Great Guardian is unable to retrieve them, like how the Moth Miraculous was assumed to be lost forever by Master Fu.
There are ways to get Miraculouses outside of being chosen by the Great Guardian. People can find Miraculouses if they have been lost and are unable to be stored properly. For example, Hawk Moth got the Moth Miraculous by finding it, also making it easy for him to utilize it for evil without Master Fu's interference or knowledge of his secret identity. Previous Miraculous holders can also give their Miraculous to someone else if they don't know the Great Guardian or the Great Guardian can't intervene first. After believing she shouldn't be the Ladybug Miraculous holder in "Stoneheart", Marinette sneaks the Ladybug Miraculous into Alya's bag in hopes that Alya will find it and take over as Ladybug, although Marinette later decides to continue as Ladybug and retrieves it before this can happen.
Another option for getting a Miraculous is stealing a Miraculous and/or removing it from its user. There are different kinds of motivations for taking another user's Miraculous. One motivation is Hawk Moth's intention to get the Ladybug Miraculous and Cat Miraculous to access their absolute power when combined. On the opposite side, if a Miraculous is being used by a villainous person, it is the Great Guardian's task to find out who the Miraculous holder is and take the Miraculous away from them. If needed, the Great Guardian will get help from other Miraculous holders or choose new Miraculous holders in order to stop the villain, Marinette and Adrien getting Miraculouses from Master Fu to defeat Hawk Moth. Along with being vulnerable to stealing if they aren't being worn, Miraculouses can be forced off the user by someone else when they are being worn, whether or not they are transformed. Despite being possible, it's less common to be able to steal the Miraculous when the holder is not transformed unless the taker knows what the Miraculous looks like when it's in its disguised form.
Not all Miraculouses owned by other people are worn and wielded by them. The Great Guardian keeps unused Miraculouses in the jewelry box in order to keep them protected and have them available for future chosen users in required situations. In a secret safe, Gabriel Agreste stores the Peacock Miraculous, a detail that goes unnoticed by Adrien and Plagg in "Volpina". At the same time, Miraculous holders who wear Miraculouses may never or rarely transform with them. Because of his age, Master Fu struggles when he attempts to transform in "Ladybug & Cat Noir", in which he hurts his back in the process. Nonetheless, Master Fu keeps and wears the Turtle Miraculous, often discussing matters with Wayzz and benefiting from Wayzz's assistance.
A Miraculous, when worn and inhabited by a kwami, transforms the wearer into a costumed appearance based on its animal theme. It also grants the wearer enhanced abilities, like strength and agility, a weapon, and a unique superpower. Some of a Miraculous' powers follow their theme, like the Ladybug Miraculous, with the power of creation, having the superpower Lucky Charm. The kinds of powers a Miraculous has aren't limited to the theme, though. The transformation lasts until the superpower is used, leaving the wearer about five minutes until they revert back to normal.
A person can wear more than one Miraculous and use them both at the same time. However, it is unknown what happens to the user when they transform, like if they take on aspects from one or both Miraculouses and if they gain more power from both Miraculouses being combined.
Jewelry that turns the wearer, with help from the pig kwami, into a pig-themed hero.
The Ladybug Miraculous and Cat Miraculous are set opposite of each other in a yin-yang section in the Miraculous box. The Cat Miraculous is placed in yang, and the Ladybug Miraculous is placed in yin.
There are only seven known Miraculouses so far, which are stored on the top level of Master Fu's chest, but Thomas Astruc explained that it's a bit more complicated than that, with more information being discovered about them throughout the three seasons of the series.
In the animatic of the theme song, the chest box has many Miraculouses that are not used in the final version, including what appears to be an Eagle Miraculous, Snake Miraculous, Bear Miraculous, Lion Miraculous, Chicken Miraculous, and Bull Miraculous.
The Snake Miraculous, in particular, is a bracelet depicting an Ouroboros, a serpent eating its own tail. It is a symbol of self-reflexivity, cyclicality, and eternal return.
Kwamis go back into their Miraculous if the holder disposes of it intentionally.
However, kwamis do not return to their Miraculous in dormant form if the holder involuntarily disposes of it, as seen in "Lady Wifi" when Cat Noir's ring falls off and he loses it.
According to what Master Fu says in "Princess Fragrance", the Miraculouses might be connected to ancient China.
The official Miraculous Ladybug Tumblr blog noted that the TV series, comic books, and web series will show the history of the Miraculouses.
In "Ladybug & Cat Noir (Origins - Part 1)", when Wayzz asks whether the new Ladybug and Cat Noir will be up to the task, Master Fu states that "[he] only got it wrong once," probably referring to him assigning a Miraculous to a wrong person in the past.
As revealed by Sébastien Thibaudeau in a press release video, in the future, Marinette will "find herself giving Miraculouses."
Also, at some point, a female character will give back a Miraculous to Ladybug, the character and the Miraculous currently being unknown.