Hi! If the card is grayed out and marked as "Enhancement", it means that the ninja is not available for enhancement in the enhancement Tab from the "Assist link" feature regardless if you have recruited the ninja or not.

If the card is grayed out and marked as "Not Obtained Yet", it means it is available for enhancement but you have not obtained the ninja.

Hopefully this helps, if you have any questions, please let us know.

Note: Not all ninjas are available for Assist link but they will be gradually added in future updates.