Ninjis are jumping black star shaped enemies that appear in many Mario games, the latest being Super Mario Advance Returns. They can be defeated with a simple bounce to the noggin. They appear in dreamlike places such as Subcon and Star Road.
Ninjis also appear in Lets-a-go, Mario, sided with Bowser instead of Wart. They attack by jumping up and down, charging at the player, and even fire shurikens and use smoke bombs to drop themselves on the player. They can be defeated by any attack, besides the stomp.