A Metamorphmagus is a person with innate shape-shifting ability in the fictional world of the Harry Potter books. Metamorphmagi are able to change their physical appearance at will. So far Tonks, a secondary character, is the only one introduced so the details are sketchy. It's known that the ability is extremely rare and can only be obtained by being born with it. Its extents remain to be seen; Tonks can make small changes, such as the length and colour of her hair and the shape of her nose, with momentary concentration and is perfectly capable of more drastic ones, such as appearing decades older than she is. Metamorphmagi presumably cannot emulate animagi and take nonhuman form, but they may be capable of passing for a specific other person. Whether or not they are capable of changing physical sex is a point that's unlikely to ever acquire canon proof.
According to plot speculation, Metamorphmagi are almost guaranteed to play a greater role in the upcoming last two books. The alternative would mean that J. K. Rowling introduced a powerful new ability and didn't use it for anything more significant than nose-changing as dinner entertainment.
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