Jaylen Kos was an Iridonian Zabrak youngling stationed at the Coruscant Jedi Temple during the Clone Wars. His best friend was Bant Eerin's Padawan. He stowed away on his friend's Jedi starfighter when the pilot was going to aid his master in the Kashyyyk system. Kos was a headstrong young Jedi trainee who resented that Ahsoka Tano was assigned to a Jedi Knight as a Padawan learner before he was.
Jaylen wields a green lightsaber like his friend before turning to the dark side.
Behind the scenes
Kos appeared only The Clone Wars: Decide Your Destiny: The Way of the Jedi and was the best friend of the character within the book that was intended to represent the reader. Based on the decisions made by the reader regarding the actions of Bant Eerin's Padawan, Kos's fate was directly influenced, for good or bad, up to 26 possible endings.
In a galaxy of ever-changing possibilities, Kos's fate was deeply influenced by the decisions of his young Jedi apprentice friend. His possible futures ranged from becoming a hero alongside his friend and gaining a Jedi Master of his own to train under to falling to the dark side of the Force and assisting Asajj Ventress.