Clark is basically Herobrine but I didn't want to use the same name and I changed his origin so it could fit into the plot.

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Clark is like a side effect or defect. Only Gareth can be "possessed" by Clark. Clark is Gareth's other personality. Clark was created from the side effects of becoming over powered and insanity. Gareth knows he exists but acts that he doesn't exist.

Gareth was one of the experiments that went haywire. The only reason Gareth was able to escape alive is because he was able to control and contain Clark. But when Gareth uses his powers or has an emotional breakdown it is easier for Clark to take control.