Prill is a little girl from the town of Milphe.
Under unknown circumstances, she was abducted by the minions of Abeloos to serve as a test subject for magical experiments. When Celica storms the wizard's base of operations, he finds her locked in a cage in the laboratory. It's unclear what, if any, experimentation she endured during her captivity, but during that time she seems to have formed an attachment to Abeloos' cat familiar.
Following her rescue, Celica returns Prill safely to her mother in Milphe.