Pixl Queen
The Pixl Queen is an unseen Pixl spoken of in Super Paper Mario. This rebellious Pixl is only talked about in four of Carson's stories, though quite a bit is revealed about her in these.
Despite the fact that the Pixl Queen was the first Pixl ever created, she seemed to stay largely quiet for what is thought to be 1,000 years after the creation of the Pixls 3,000 years before the setting of Super Paper Mario. However, seemingly long after the passing of her creator (the powerful magician who invented the process of Pixl-making), the Pixl Queen (then of an unknown name) finally rose into power, renaming herself with her only known name of "Pixl Queen". She then began to use her powers of mind control to manipulate all Pixls, causing them to eventually enslave most of the Tribe of Ancients (who were ironically once their masters) in her name. However, the owners of the magician's twelve original Pixl creations managed to somehow escape this Pixl Uprising and started to fight against the rebellious Pixls, containing them within Catch Cards and healing them. In the end, only one of these heroic Ancients survived, and he traveled with only his Pixl friends to the Pixl Queen's castle. Once he arrived at the stronghold, he confronted the dark Pixl Queen herself, who immediately hid from the hero and shot a barrage of disturbing images to his mind telepathically. Although this greatly weakened the Ancient hero, one of his Pixls helped him through the tragedy and ended up defeating the Pixl Queen itself, although it was destroyed in the process of doing so. Afterwards, it was shockingly revealed to the hero who owned the Pixl that the Pixl Queen was the first created of the Pixls (perhaps created even earlier than the magician's so-called "original" twelve Pixls) and that the magician's motives for creating her involved the study of the Dark Prognosticus. The Ancient survivor, attempting to stop such a tragedy as the Pixl Uprising from ever happening again, then took the Dark Prognosticus with himself to unknown parts, likely founding the Tribe of Darkness known to once have possessed the dark book. Afterwards, the Pixl Queen's mutinous efforts led to the fall of the Tribe of Ancients and the outlawing of Pixl creation.
Though these are the only know actions of the seemingly late Pixl Queen, more is indeed known about the Pixl herself. Although the spirit united with the Pixl vessel that formed the Pixl Queen had previously been described as "demonic", Carson later stated that a certain one of the Pixl-creating magician's books revealed that the Pixl Queen's spirit was actually derived from a human rather than a demon. It was on this principle that Carson theorized that the Pixl Queen's spirit may have been that of the magician's own daughter (who did indeed get a Game Over during her father's life due to a deathly illness), although he later seemed to have disproved this theory in his statement that Pixls' spirits cannot be derived from Game Overed people. It is also possible that Mario, Peach, Bowser, and Luigi did indeed meet the Pixl Queen in some form on their adventure; it is apparently possible that Mimi is a Pixl and she, like the Pixl Queen, is both female and evil. It is also a (perhaps more likely) possibility that Shadoo is some sort of revived form of the Pixl Queen, as Shadoo is also a rebellious creation of the Ancients who speaks of once using the power of the Pixls to get revenge on his Ancient creators. It is also unknown if the Pixl Queen had any helpful ability, although she was indeed able to control all Pixls and used the power of telepathy in her battle against the Ancient survivor, making these possible ability candidates.