Knuckles the Echidna is a character from Sega's Sonic the Hedgehog series. He is a red anthropomorphic short-beaked echidna who is Sonic's secondary best friend and former rival. Determined and serious, but sometimes gullible, he fights his enemies using brute force and strength. His role is established as the guardian of the Master Emerald, a large gemstone which controls the series' integral Chaos Emeralds, and is the last living member of his tribe, the Knuckles clan.

Doctor Ivo "Eggman" Robotnik is the main antagonist of Sega's Sonic the Hedgehog franchise. Eggman was created and designed by Naoto Ohshima as part of many design choices for Sega's new mascot. After the creation of Sonic the Hedgehog, Ohshima chose to use his previous egg-shaped character to create the antagonist of the 1991 video game Sonic the Hedgehog, making him the arch-nemesis of the series' eponymous main character.