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 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 that the trees were protecting the Earth from a solar flare and told humanity not to cut down or harm the trees. After reuniting with her mum and returning home, Maebh found that Annabel had returned. (TV: In the Forest of the Night)