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Mordicus Egg was a renowned expert on Muggle society in the British wizarding community as well as a former headmaster of Hogwarts School of Witchcraft and Wizardry. Even though he was raised by his pure-blood parents, Egg was fascinated by all aspects of Muggle culture and how they were able to do tasks without the aid of magic. After he attended and graduated from Hogwarts, Egg took it upon himself to do field studies and blend in with common Muggles in Liverpool for around a year. He realized not too long after beginning his initial study that he needed to broaden his horizons, so he then left Liverpool and began to travel the world to study how Muggles interacted with one another in different cultures. Excited to share his findings with Ministry officials, Egg traveled back to England and worked in the Muggle Liaison office for several years.

Egg left his job at the Ministry when a position for Muggles Studies professor opened up at Hogwarts. He taught for a short while before he was promoted to headmaster of the school. Egg advocated equality and positivity during his tenure as headmaster. Instead of only punishing students who mistreated Muggle-borns, Egg took it upon himself to have private counseling sessions in his office to educate students on their behavior. Eventually, the number of counseling sessions along with the duties of being the headmaster began to take a toll on Egg. He found the job to be too stressful and resigned. After his resignation, Egg wrote two books that were based on what he learned from his travels: The Philosophy of the Mundane: Why Muggles Prefer Not to Know and Cooking the Muggle Way. Both books were well received by the public and have occasionally been used as optional textbooks for Muggle Studies.

There are two different portraits of Egg at Hogwarts. One of them hangs in the Grand Staircase under the third floor landing, depicting Egg when he was younger, presumably while he still worked at the Ministry. The other is a portrait of a much older man, which resembles Egg towards the end of his time as headmaster. Both portraits are very enthusiastic, especially when the subject of the conversation is on Muggles. Egg also will not hesitate giving advice to those who are bullied, and will often attempt to cheer them up.