Apep is a minor character in Mortal Kombat: Deception.
This young boy can be seen in the beginning of Deception's Konquest Mode. He is Shujinko's guide and one of Bo' Rai Cho's students who also gives you several hints. He is not seen again after you clear the village portion of Konquest.
As he can be seen in the game, he may have reached Shujinko's age, barring anything that may have happened to him (killed during an invasion of Earthrealm, etc.). This, however, is never mentioned in the game.
In Mortal Kombat X, Apep is mentioned in the "Fallen Friend" premiere tower where Apep was killed by Quan Chi, and Bo Rai' Cho has to free his soul.