From Melissa Marr's website:
(Bar-a or Beer a)-- a version of the character Cailleach Bheur/Bheirre/Beara/Bheara/Birra/Beirre from folklore. Beira is the Winter Queen, but the myth behind her is too complex for a simple definition.

Job: Winter Queen.

Status: Dead.

Description: For the past fifty years or so, Beira has been wearing a flowery dress and apron with a necklace of pearls, in a mockery of 1950's motherhood. Her dark hair is worn in a chignon. This is a glamour.

Underneath the glamour, her eyes are filled with snow and her lips sparkle with frost. Like Donia, the Winter Girl, cold surrounds the Winter Queen. The Winter Girl is filled with cold and can't control it. The Winter Queen IS cold, and has the ability to control it, if she wishes.

With or without her glamour, Beira moves with grace and elegance. Melissa Marr has described her as "lovely, & a bit mad".

Personality: Just as she does with her appearance, Beira likes to "put on" a sickly sweet motherly attitude. Her concern for Keenan is a way to mock him over the "game" of finding the Summer Queen.

In reality, she is brutal and vicious. She seems to enjoy destroying things. She deliberately hurts those closest to her (her wood sprites are described as being bruised). She kills a hag, threatens to kill Donia and does severe harm to Keenan on more than one occasion. Her overall plan is for Keenan's powers to fade as he fails to find his Summer Queen to the point she can destroy his court and spread Winter's influence throughout the year.

The death of mortals is neither a motivation nor a deterrent in this plan. Her court will survive; the rest is immaterial.

History: It's not clear how long Beira has been Winter Queen. There's no indication there was another before her. At some point, she and Keenan's father, also the Summer King, entered into some sort of relationship. She killed him the night Keenan was conceived.

It's not clear if she knew Keenan would be Summer King before he was born, but she knew by the time he arrived. She and Irial, the King of the Dark Court, bound Keenan's power, cutting it in half and sending the missing part out into the world, where he would have to search for the mortal girl who could claim it and become his Summer Queen.