A young prostitute at Chataya’s brothel, Mhaegen is a young girl—certainly less than 16 years of age—who slept once with the king, probably while still a virgin. She has red hair and a dusting of freckles.
In the novel, the girl’s name is never given, but for the series one was created by George R.R. Martin. The actress cast to play her is said to pass for 20 or so, so she seems to have been aged up somewhat from the book.