Cool Cool Bye (Series)

Two brothers are riding around on their hover bikes, when suddenly a giant metal fortress gets in their way. After a brief battle, the duo manages to gravely wound the fortress. It retreats and dumps our heroic pair in a wasteland. But this wasteland is inhabited by nomads that indicate that the fortress was hidden inside a mountain. The nomads plead for the duo to finish the job. Two of the natives go with them as guides, and somewhere along the way the gain the help of a fairy that can transform into a mech-warrior woman. (Source: irreality.org.uk)