He is the evil counterpart of Jake Long. He is accidentally created by Jake and attempts to take his place by absorbing his chi energy. He only appears in the episode "The Doppelganger Gang" in Season 2 and never makes another appearance.
Taught by his grandfather in how to make nearly perfect replicas of himself using his dragon chi, Jake took disadvantage of the ability, creating far more than the mission at hand required.
When challenged by Brad to a fight, Jake decided to send in a chi-clone as a ringer so he would not have to take the lumps himself. However Jake made use of a dangerous artifact which ended up instilling Evil into the doppelganger, which immediately decided it had a better idea than following Original Jake's orders.
As a Near-perfect chi duplicate, Evil Jake could pass as Jake's twin brother, looking exactly like his creator but slightly darker in colour.
The evil doppelganger then proceeded to absorb into himself the other fake Jakes in order to build up and reinforce his own store of chi power.
Eventually it was Evil Jake which itself got reabsorbed back into Original Jake.
Evil Jake looks exactly like the original Jake, but with slightly darker colors. The evil doppelganger eventually goes on to absorb the other Jake doppelgangers in an attempt to overpower the real Jake. In the end, Jake reabsorbs Evil Jake's ch'i, possibly destroying him.
- While "Yang Jake" apparently was used in production and script to differentiate from Good Jake, as the name was never used in the show, this is a rare case of Official yet Non-Canon.