The Forever Ninjas have dedicated themselves to upstaging the Infinity Knights by any underhand trick. They are not on the level, but sometimes Len 10 and the gang have to match force with the Ninjas to defeat the Infinity Knights.
The Forever Ninjas began after the Negative Ten was defeated. The Forever Ninja decided start off on his own and traveled around the world enlisting top acrobats to join him and train as ninjas. Now the desendents of those acrobats have become the most precise and daring team of thieves and smugglers ever.