How about a clan's villiage destroyed by imperial forces for speaking outa gainst the Empire/Mythar. Last surviver of the Ninja school the villiage supports is cast out (or leaves) with a serious grudge against the Empire and the Mythar respectivley? If I can't make this concept work, I'll scrap it and scoop something else out of the swirling maelstrom of my brain.
Edit: Brainstorm. It seems that the Zodaku are deep into spirit magic. How would it seems if a meditatiing Ninja were given a vision of the Mythar? Especially if that vision (true or not) were of a Mythar betrayal. He'd take it seriously I assume, and after expressing concerns to the Elders at the school, either be told to seek out the truth or be kicked out immediatly from fear of reprisal or Anti-Mythar sentiment.
Edit the second: Brainstorm Mk. II. His Grandfather and several other Ninja were hired by the Agorian Army, perhaps as a strike force to get behind the enemy lines and take out the tribal chiefs and cause trouble, after the Mythar incursion and his grandfather's death it became a blood feud. When he reached appropriate training and age his father left to fight the Mythar and never returned. Now (I haven't figured out a name yet) faollows his father's footsteps.