Thousands of years old, in the body of a twelve year old, and extremely tired of being told she is adorable.
Sprite can project lifelike illusions, and she is stuck in a permanent childlike state, appearing as a young girl despite being thousands of years old. She often uses her power to appear as an adult woman called Sandra.
Hers is the cruellest arrangement in the film and the one it is most careful with. She has an adult’s mind and seven millennia of experience and nobody has ever treated her as anything but a child, and she cannot be touched, loved or taken seriously.
Minor spoiler
At the end Sersi uses what is left of the Uni-Mind to make Sprite human, which ends the permanent childhood and starts the clock. It is offered as a kindness and the film knows it is also a death sentence.