Realistic Ninja game: work in a rice field for twenty years. A samurai back-hands your mother. Go to his home and stab him with a wood-carving knife in the middle of the night. Tell no one. Resume your life of farming.
Fun Ninja game: you have a sword that can slice through an aircraft carrier. You know this through experience. Fight an asortment of demons, robots, and nameless soldiers. Your female accomplice has two fitness balls hanging from her chest. And right now you're flying miles above the ground, but you're not exactly sure how or why.