That's an interesting idea, I like it. What about polymorphing the Ninja into an intelligent animal somehow? That could be a driving force for him to follow your hook...he has a chance to get turned back into his normal form if he accomplishes the task.
That idea is absolutely amazing. I like it much.
Though a few ideas for the Ninja:
*Ninja finds an item with hidden semi-beneficial curse that turns them into an animal.
*Enemy spellcaster polymorphs Ninja into animal, with a fix found after resolution.
*Ninja become Lycanthrope, but has trouble changing back due to the artifact
*Perhaps a sneaky* breed of animal likes and imprints on the Ninja.
*Ninja finds a magic item that grants a familiar or animal companion**.
...I hope at least one of those ideas could work for your campaign.
*Perhaps a raccoon, ferret or :\ a recently deceased individual's pet montrous centipede...
**This creature could be magically linked to the item.