Mimicry is not in itself a Special Ability, but it is a skill that can be learned. Polyglots tend to be good at mimicry, but other elves, like Keefe, can also learn. It can be noted that Sophie is not the best at micicry, and when trying to sound like Lady Cadence, she ended up sounding like a 'grumpy Mary Poppins'. sometimes, it takes lots of practice.
Known Elves That Can Mimic
He proves that he can do uncanny impressions of various elves, including Sophie, Lady Gisela, Magnate Leto, and Dame Alina. Keefe uses this to mimic Sophie, summon the cache from the void in Neverseen, try to trick Gethen in Neverseen, get out of detention, and even just to annoy his friends.
She mocks Sophie during her class, using many different voices. She also tried to teach Sophie to mimic, using an unknown golden gadget shaped like a metronome to measure her scores. Sophie apparently scored lower in mimicry than some toddlers, her first attempt getting her a score of 3,400,753.
Keefe has said multiple times that he had learned the art of mimicking from his mother. Lady Gisela also mimicked Lord Cassius during the battle in Mount Everest, to trick the Keefe and his friends into thinking that he (Lord Cassius) was part of the Neverseen, though it was her the entire time.