After being found alive in Glendower's false grave she moves into 300 Fox Way.
Gwenllian was born to an old tree-light (unnamed in the books) and the prince of Powys, Owain Glyn Dwr (Glendower). Seeing that her mother was a tree-light, and thus not Glendower's wife Marred (Margaret), she was legally illegitimate.
She did not approve of her father's poet and adviser, as she believed that he was feeding her father lies and manipulating him, so she tried to stab him at dinner one night.
This lead to her being used as the bait in Glendower's false grave, set up in the cave in Cabeswater to distract from the real grave of Owen Glendower in Virginia.
When Gwenllian was put into the grave, she was tied facing downward, with her hands behind her back, a burial style seen in suicides, adulterers, and witches of the time. She was "put to sleep", but she never actually slept. Instead, she stayed awake for six hundred years until she was found by Blue, Gansey, Ronan, and Adam. They brought Gwenllian out of the cave and back to 300 Fox Way with them, where she ended up staying.
Gwenllian describes herself as a "mirror" and a witch. This means that she can amplify and reflect magick and energy off of herself, very similarly to Blue's abilities.
Gwenllian has an affinity for singing, whispering, and shrieking instead of talking. We don't really know why.