Maebh Arden

 was a student at Coal Hill School. She had lost her sister, Annabel, after she went missing. She was able to communicate with the trees after the Earth was turned into a forest. With the help of the Twelfth Doctor she 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.