At this stage, it is unknown who the 'witch who isn't a witch' is, why they awoke the dragons or why they are described as such. Besides the description given by Azhdeha (who was killed soon after) no other clues have been given to person's identity or motives. Presumably, they have powerful magic, if they are able to awaken the dragons and have some ties to the witch species, but may not necessarily be a witch. It is likely that the witch who isn't a witch will be identified in the final novel, Strange Fate.
Possible candidates Edit
Maya was originally a witch, but used a spell to transform herself into the first vampire. Maya was therefore technically no longer a witch, though she retained her witch powers. Maya was certainly an exceptionally powerful witch and may also have been alive during the witches' war with the shapeshifters (given her title, Maya 'Dragonslayer'), and so may have knowledge on the spell used to put the dragons to sleep. Maya was disdainful of the witches, believing vampires, the race she created, to be superior and held a grudge against her sister, Hellewise, whom she attempted to usurp control of the witch tribes from. Therefore, it wouldn't be much of a stretch to imagine she would further betray the witches by awakening their oldest enemy. The only problem with this is that Maya was killed in Soulmate, several months before the events of Witchlight, though it's possible Maya had awoken Azhdeha at an earlier stage and he had simply been laying low and biding his time.
Mal is said to be a vampire-witch Hybrid, the son of a Harman and a Redfern. True Hybrids are shown to possess the traits of both their parents' species, but are not truly one or the other. It's highly possible Mal, as a Hybrid, can use magic without being a true witch (due to also being a vampire) and therefore could've awoken the dragons. That being said, Mal is a member of Circle Daybreak, thus making it odd that he would awaken one of the Circle's enemies.
An unlikely, but still possible candidate. Sylvia was a witch and descended from the Harmans on her father's side. She resented the Harmans for rejecting her and chose to remain loyal to the Night World after the witch clans seceded to Circle Daybreak. Sylvia answered to Hunter Redfern, who saw the upcoming apocalypse as a chance for vampires to gain control of the world and so could've ordered Sylvia to awaken the dragons. Sylvia was shown to be a very powerful witch, capable of creating a magical arm brace that suppressed even a Wild Power's blue fire. Given that Sylvia is killed at the end of Black Dawn, which takes place not too long before the events of Witchlight, Sylvia could well have had the opportunity to awaken Azhdeha. Sylvia, after turning her back on the witches, declared she was "no longer a witch", effectively disowning her heritage, which could pertain to Azhdeha's statement (seeing as he may not have known about Sylvia's change of heart). However, despite Sylvia's claim, she was always technically still a witch.
Hunter, as previously mentioned, saw the apocalypse as an opportunity for vampires to become the most powerful species on earth again and enslave the other races. Hunter would certainly seem like the type of person who would resurrect the dragons for a chance to grab power and destroy the witches. Hunter is also technically part-witch, because of his blood-tie with Maeve Harman (and possibly his relation to Maya, his ancestress who was originally a witch and could still use her powers after her transformation). However, Hunter never displays any ability to use magic and as far as is known, the only witch who became a vampire and could use magic was Maya, who, due to her unique status as the first vampire, may have been the exception. Thus, the chances of Hunter being the witch who isn't a witch are slim.