Unknown, though she is at least 1,500 years old, having vague memories of the rise and fall of Dal Riata, so she is from at the very least the 5th century C.E.
Possibly even older, due to her finding records of herself or someone very much like her dating back to 0 A.D. and even earlier

They present as female, though Blaire has been alive so long that they have actually forgotten what they used to be

Witch, the Woman in the Mists,
the Blood Witch, The Witch of the Mist
One Who Brings Death

Main Character

They were once pale blue - At least, Blaire thinks they were. Usually nowadays, they are gold, though they do change quite a bit depending on what magicks are performed. Or their mood. Or the day. Or if the planets align. Or... you get the idea.

They are partial to twirled moustaches when appearing as a male

Their hair is usually shoulder length and combed with the bangs swept to one side, though they have been known to use spells to change it according to their whims.

They change their hair a lot, but often it is jet black with a dark green tint to it when light shines off it.

Blaire's height is variable, though they tend to maintain it around 5'9"- 5'11"

Mole under their right eye.

Whatever they feel like it to be. As a witch, Blaire can change it at will.

HAHAHA. That's like asking if the cat in Schroedinger's box is alive. There is no way to answer the question. Blaire will literally make themselves weigh more than an elephant if they want to, or as little as a mouse if given the inclinations to.

Again, variable, though they tend to keep it pale. In some of Blaire's earliest memories, they have darker skin however.

witch????? Blaire doesn't even know themselves. Were they human once? Did they make a deal with some supernatural being?

Magic. Blaire IS a witch, after all.

Blaire tends to speak in rather archaic terms towards everyone, an indication of just how old they are. They also tend to call everyone, even old men and women and other witches, save a select few, as children. Blaire will also ask for people's firstborn child whenever negotiating prices for her services (jokingly of course. No one actually has uses for the firstborn children anymore in witchcraft).

What motivates Blaire? That's kind of like asking what motivates the sun to rise, or what motivates life to exist in the first place. The short answer is, Blaire is motivated by whatever catches their eye or is on their mind at the time. The long answer is that no one really knows what motives them. On the one hand, she aided the Soviets in World War 2, but on the other, she aided Christians during the Crusades, and Joan of Arc, and the Spanish Inquisition, and the Templars, and Genghis Khan, and so on and so forth.

They are Arrogant, Lustful, Slothful, and very flippant. They also love booze. A lot.

Blaire isn't really prejudiced. They've been alive long enough to see almost everything, and honestly? Blaire doesn't grasp how or why humans would have prejudices against each other.

Flying is a large hobby of theirs. Often Blaire can be found cruising around the skies either on their broom, carpet, or any of their other enchanted belongings. Sometimes they actually fly around on the outside of passenger airliners or military craft (though not as much as they used to. Getting shot at by Airmen gets annoying fast.)

Blaire also likes to hang from tree branches by their legs for long periods of time, as well as sleeping.

She can outdrink almost any person alive on the planet, save one. It's not a good idea to beat the God of drinking at his own game....

Blaire is very mellow, tending to just go with the flow in most situations. Not much actually gets to them.
If they take a liking to you, good luck in trying to get rid of them until they tire of you. Blaire is always up for parties, and even partakes of recreational drugs when she is offered them, though they don't really do much to them other than make them a little more chatty.
Always down for hanky panky because why not? it's not like it's hurting anyone.

Emerald Green and Royal Purple are their two most preferred colours

They never understood the fascination for cats or dogs as familiars. In fact, their favorite animals are the tarantula and vulture.

Blaire literally uses magic. They have no need for weapons

Blaire doesn't have just one favorite possession -they have many. Most notably, their Grimoire, wand, and Nokia

Beer.

Witch, Freeloader, Immortal being

Not only does Blaire not like politics, They also despise nearly all politicians, mainly due to knowing how corrupt they all are, considering she has granted many, many of them favors in exchange for money or other services....

They like to claim being atheist, though in reality they know SEVERAL gods. Blaire just likes to piss some of them off, so they say that they don't believe in god.

Officially, she is a jobless bum who somehow has a LOT of money. Unofficially... well, Blaire doesn't have a job

Whenever Blaire feels like it.

Good luck getting that out of them. Blaire doesn't even know

Blaire is largely self taught, though they did study under Merlin and then Morrigan for a few years/decades when learning to perfect their magic.
In terms of standard education, occasionally, Blaire will go through university or high school (complete with forged records) to brush up of non-magic topics

Blaire has an immortal Tarantula named Petyr, a Burmese Python familiar that calls itself Jeffery, and two vultures -an Egyptian Vulture named Baba, and a Bearded Vulture named Asha

Blaire is one of a few extremely rare types of witches.
Their magic, while most openly displayed via incantations and/or use of their grimoire, is actually unconsciously activated -for the most part. After having lived long enough, Blaire has great control over it, being able to funnel their magic into all three types; spoken, unspoken, and emotional.
Their magic falls into five separate class types: Elemental magic (primarily the Fire Element for things like fireballs, explosions, etc.), decay magic (different from black magic or necromancy. Decay magic only decays things, though sometimes to the point of death), Healing magic, summoning, and wind magic. They mostly use Decay and Wind magic, however, as they are 'more fun to use' (plus, when used properly, they can make a whole room smell like old books. Who doesn't love that smell?)
Unlike other witches and warlocks, Blaire is not registered on any list of magi by humanity. It's not that they aren't known to the world governments -it's more that there isn't a record of Blaire's birth OR death, so there is nothing for them to place on the list, other than 'unknown magic user.'

Blaire tends to stay away from Britain due to their fragmented memories of living in Scotland and seeing the (then) invading British razing their home at one point. They prefer to spend their time just traveling the world, sleeping wherever they feel like, eating whatever they want.

Of note: Blaire has a mysterious connection to a person called Ummashtart Ummashtart