Maebh Arden

 was a student at Coal Hill School. Her elder sister, Annabel, went missing. This was presumed by many to be the cause of her mental distress. Maebh was then able to psychically communicate with the trees after the Earth was turned into a forest. She was also able to pick up thoughts from other sources, such as a desire to contact the Doctor from Clara Oswald. With the help of the Twelfth Doctor, Maebh was able to warn the human race before a solar flare hit the Earth. (TV: In the Forest of the Night)

Behind the scenes
Maebh's surname, Arden, refers to the Forest of Arden in William Shakespeare's.