Mok works for The Government. He experimented on the gifted that were captured and after witnessing the power of some of the more demonic creatures, he decided he wanted to harness that power for himself. He's greedy and power hungry, which is why he created Angel and Omar in secret.
Angel and Omar call him father, although he never loved them as his children, he only loved the power that they would bring him.
Mok is seen In an memory of Jack Skellington