The Familiar of Zero (ゼロの使い魔 Zero no Tsukaima) is a fantasy and comedy-oriented series of Japanese light novels written by Noboru Yamaguchi and illustrated by Eiji Usatsuka. The story features several characters from the second year class of a magic academy in a fictional magical world with the main characters being the inept mage Louise and her familiar from Earth, Saito Hiraga. The series has been adapted three times as an anime series.
Plot Summary (Anime): Meet Louise, a budding magician. The students at the Tristein Academy call her "Zero Louise", due to her current record of zero successes with magic. In fact, her magic tends to go spectacularly wrong. Now, as an important test of her aptitude for magic, she must summon a familiar to be her servant. At this critical moment, she summons up all her magic and wishes for a familiar that is "devoted, beautiful and powerful", and gets... Hiraga Saito, an ordinary Japanese boy. It's difficult to say who is more surprised and dismayed, but the rules don't allow for second attempts. Louise is stuck with her strange familiar, and he with her.
(source: Wikipedia, ANN)

Related anime: Zero no Tsukaima: Futatsuki no Kishi, Zero no Tsukaima: Princess no Rondo