Cobra-Kai. I've heard about you.
—Davey Wilson and Cobra-Kai
He was known for using his skills as a ninja to cause pain and misery among the people of Ninja City, along with multiple other cities, as well as his participation in the Battle of the Two Ninjas.
Cobra-Kai had black hair, brown eyes, and an outfit consisting mainly of black similar to Davey Wilson's. He also wore a black eye mask and was in possession of a legendary golden sword which he used in the Battle of the Two Ninjas.
Cobra-Kai was born as Kai Ryan Cobrin to unknown parents. All his life he had an interest in martial arts, earning himself the nickname "Cobra-Kai" based off of his name and the character in the movie Karate Kid.
After his training was complete, he went only by Cobra-Kai, and despite the Grand Master's urging not to, he used his skills to inflict pain and suffering upon the citizens of Ninja City.
Battle of the Two Ninjas
The Battle of the Two Ninjas was a duel of ninjutsu between Davey Wilson and Cobra-Kai resulting in Cobra-Kai's death due to inhalation of toxic fumes released by Davey.
The exact details of the battle can be read here.