My normal is on a ninja that I have 3 star of (Mabui, common from kage)
My rare is on a ninja I don't even have, Ningendo, rare from tendo treasure.

That more or less answer all your questions. Thou the rare vs common issue might be problematic on ninjas that were rare on lower treasure (sealed treasure, for example) that becomes common in later treasure. Thou I would assume that it would be classified as common, or it would be REALLY annoying.