Mahou Shoujo Lyrical Nanoha

The story line follows Nanoha Takamachi, an everyday nine-year-old Japanese girl. Nanoha's regular everyday life ends when she rescues an injured ferret who reveals himself as a young shapeshifting mage named Yuuno Scrya. An archaeologist from a parallel universe, Yuuno came to Earth to collect a set of twenty-one dangerous ancient artifacts named the "Jewel Seeds" that he first discovered in his own world. Jewel Seeds give living beings who come into contact with them unnatural powers, often turning them into monsters, and Yuuno, injured while trying to collect them, must now rely on Nanoha while he convalesces in ferret form.

(Source: Wikipedia)