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Dilys Derwent is known as one of the most celebrated headmasters along with Albus Dumbledore and Everard. She worked as a healer at St. Mungo's Hospital for Magical Maladies and Injuries in 1722 before becoming headmistress of Hogwarts School of Witchcraft and Wizardry in 1741. Derwent was very popular amongst the students and faculty. She made several contributions to the Herbology and Potions departments. She was also regarded as one of the top healers in St. Mungo’s when she worked there. Whenever the hospital wing was backed up with several students, Derwent would not hesitate in helping the school matron treat them. She was regarded as motherly, as she often asked students about their day while passing by in the corridors. If it appeared that a student was not having a good day or that they were sick, Derwent was genuinely concerned for their wellbeing. She died in 1768 while serving as headmistress.

Professor Derwent's portrait is located not only in the Headmaster's office, but in the main admitting area of St. Mungo's Hospital for Magical Maladies and Injuries as well. Due to having her portrait in two different areas, she can easily travel back and forth. She is very helpful towards headmasters and the staff at St. Mungo's. Derwent typically keeps the current headmaster updated on the conditions of notable patients by traveling back and forth between her portraits. She is known to give encouragement to the staff of St. Mungo's whenever they feel stumped on severe cases. Sometimes, she is willing to hint at which potion or spell to use when the healer she is talking to truly feels lost.