For the story, see Doctor Sun (story).
Doctor Sun was the leader of a cult specialising in keeping children safe from the regime of Doctor Robotnik. In reality, Doctor Sun was Robotnik, disguising himself as a kindly sage keen to protect young Mobians. Based in the Mystic Cave Zone, Sun encouraged victimised children to stay in his hideaway, enticing them with a library full of comics and a recreation room with all the latest vidoe games. Once they felt they were ready to join Sun's order, they could enter the Salvation Doorway.
Upon hearing from Mr. Stripes that his son Stripes had joined the cult, Sonic the Hedgehog and Tails headed to rescue him. They learned of Doctor Sun's ways and Sonic was about to accept the children's free will when Sun uttered the word "Eggsactly", clueing Sonic in to him being Robotnik. After ripping off his disguise, Robotnik was ousted and the "Salvation Doorway" was shown to lead to a Badnik Processing Plant, that Sonic promptly destroyed.