Metamorphose

Metamorphose (肉体変化 (メタモルフォーゼ) ), also known as Metamorphosis in the Baka-Tsuki and Yen Press translations, is an esper ability that is purported to exist in Academy City. As it name states, it is a power that allows the user to change their appearance.

Principles
According to Etzali, the ability allows someone to have a face or body similar to others, though it is apparently impossible to change in a genetic level. It is probably similar to Aztec's magic, which uses the flaying of skin to use a disguise.

Tsukuyomi Komoe references that said ability is rare, and that only three espers in the whole of Academy City have that ability.

Three Stories Arc
Etzali first mentions the ability to make Kamijou Touma believe the real Unabara Mitsuki was an imposter using this power to mimick him. Later, after his true identity is exposed, he adds that his power isn't Metamorphose but similar to it, cluing Touma into the fact that he is a Magician.

Kazakiri Hyouka Arc
When answering Kamijou Touma's question about what esper ability could cause Kazakiri Hyouka's condition and strange body, Tsukuyomi Komoe mentions espers with the Metamorphose power as one possible explanations, but shoots down Touma's conclusion that Hyouka is such an esper, as she isn't any of the only three espers possessing it.

Kamijou Arc
When faced with the other Touma, Misaka Mikoto considered Metamorphose as a possibility.