As the guru in Cephiro, he provides people with magical potential with magic and teaches the spells to them. He promises Princess Emeraude to help lead those from another world to become the Magic Knights. He is only a few people in Cephiro who knows about the truth about the legend of the Magic Knights.
Although Clef looks like a little kid, he is actually 745 years old (looks can be deceiving, isn't it?). He also has a short temper. He is also the one giving the knights their armor and their magic.