Shadow Jago is a character introduced in Killer Instinct as a character but an evil version of Jago.
Although his moveset is identical to Jago's, the visual effects are differed in order to reflect Shadow Jago better (the fire in Jago's Endokuken is changed to a black, shadowy effect, etc.).
His voice is also more sinister than Jago's. Even though his voice is different, both Jagos share the same quotes.