Mieu is a small blue mammalian called a cheagle. Cheagles are considered sacred by the Order of Lorelei due to a pact formed with Yulia Jue, symbolized by the Sorcerer's Ring. The ring enables cheagles to communicate with others, as well as use the power of various fonons to gain new skills.

He joins Luke's group when the elder cheagle exiles him for setting fire to the forest where ligers live, which causes the ligers to hunt the cheagles for food. Since Luke saved Mieu's life, the elder orders him to serve Luke for the time of one year.

Source:Wikipedia