Sega's Sonic CD is remembered today for many things including its divisive soundtracks and the time travel mechanic that turned every stage into three stages, but I would say that the most memorable addition to Sonic's world is the creation of Metal Sonic. Sonic's robotic rival gave him someone he could evenly spar against with speed versus speed. The character is designed to look menacing and just plain painful to touch with his sharp edges and glowing red eyes. What went into designing Metal Sonic? Over at Shmuplations there are translated interviews with Metal Sonic's creator. No, not Dr. Robotnik, but Designer Kazuyuki Hoshino as originally presented in the liner notes to the game's soundtrack album.
When I first heard the words “Sonic’s Arch-rival” and “Sonic’s Doppelganger” in the design notes, an image for that character’s design immediately came to me, in almost complete form. Metal naturally fit into our key visual concept for Sonic CD as well, and from the first moment that I imagined his red iris set against the darkness of his black eyes, I knew he would become a character with real, lasting appeal.
This was something I thought about later when I designed Shadow the Hedgehog, too, but seeing as Metal Sonic was a rival character to Sonic, I knew the best way to show that off would be with an in-game scene where Sonic and Metal Sonic directly compete with each other—and I designed Metal Sonic with that scene in mind.
The race through Stardust Speedway Zone 3's bad future area is the most memorable moment in Sonic CD, so much so that it's recurs several more times over the course of the franchise including in games like Sonic Generations, Sonic the Hedgehog 4: Episode II, and Sonic Mania. Metal Sonic is my favorite of the Sonic villains because he's singular in purpose, doesn't care to perform long angsty monologues, and is driven to exceed his limitations while simultaneously holding himself to Sonic's example. He wants to match Sonic, but also surpass him. It's an interesting duality.