Ninja is an advanced version of the standard Bear. It can teleport to any space, randomly. The Ninja cannot be trapped. It can only turn into a Tombstone when you use an Imperial Bot on it. It is effective at blocking where you place Items. It can easily ruin a strategy. As with the "Peaceful Valley", there are no Ninjas on "City on an Island".
If there are any Ninjas in your town when the last empty space is filled, they will turn into Tombstones. It is possible for these Tombstones to be a part of a combo that forms a Church. As this frees up space, you will be able to continue adding to your town.