He had a 0.01% chance to spawn in SR3. That's not a guaranteed spawn for every 10,000 peds, that's every ped having a 0.01% chance to be Genki. And this was per island, so your odds decreased even more if you crossed bridges frequently.
Fun story: Very first non-dev I saw playing the game was release night at GameStop. He got to the "Buy a Gun" tutorial and decided he wanted the Penetrator so he went outside to mug peds. Genki was waiting... poor guy only had a pistol and like a hundred rounds. Spent about ten minutes kiting him before he got him down. The entire store cheered.