Metal Sonic in Sonic the Hedgehog
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Parent game

Sonic the Hedgehog is a 2D side-scrolling platformer developed by Sonic Team and published by Sega for the Genesis/Mega Drive. Players control Sonic the Hedgehog, who can run at near supersonic speeds and sets out on a quest to defeat Dr. Robotnik, a scientist who has imprisoned animals in robots and seeks the powerful Chaos Emeralds.
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Metal Sonic in Sonic the Hedgehog

A Hack of Sonic the Hedgehog, fully replacing Sonic into a playable Metal Sonic, along with REV01 background effects and a few tweaks. comes with 4 different playstyles for Metal Sonic.

Metal Sonic in Sonic 3 & Knuckles replaces Sonic with Metal Sonic from Sonic CD, you can no longer choose from the numerous playstyles but Metal has his own abilities such as use of the elemental sheilds and a boost. Metal Sonic must save Angel Island and take down his Mechanical Doppleganger.

Metal Sonic in Sonic the Hedgehog 2 swaps Sonic out for Metal Sonic from Sonic CD, you can choose between the Original moveset, Sonic CD, or the 2011 remaster moveset from Christian Whitehead's port of Sonic CD Metal will once show Robotnik he's not a bot to be forgotten...
Could be interesting

Crackle Cradle is a 2D sidescrolling platformer/shooter developed by FoxTail. This game entails guiding the selected heroine through various areas, traps, and monsters, relying on a mixture of evasive skills and accuracy with their weapons.

A multiplayer-only ROM hack of Sonic the Hedgehog. Two players select a character (one being Sonic and the other being Tails) and both battle it out to try to gain as many hits on Eggman. The catch is that Eggman only gets destroyed when the timer runs down to 0 (unless it's a draw at that time, then, it will wait for another hit). The player with the most hits at the end of the timer wins. The player can also win if the other player dies before the timer runs out (if respawn mode is off).

A challenging puzzle platformer inspired by games like Limbo and Badland.

Forgotton Anne is a 2d cinematic adventure game combining puzzle platforming with adventure game elements. You play as Anne, the enforcer keeping order in the Forgotton Realm, as she sets out to squash a rebellion that might prevent her master, Bonku, and herself from returning to the human world. The World of Forgotton Anne: Imagine a place where everything that is lost and forgotten goes; old toys, letters, single socks. The Forgotten Realm is a magical world inhabited by Forgotlings, creatures composed of mislaid objects longing to be remembered again.

Planet Alpha is an adventure set in a living alien world where you have the ability to manipulate the daytime. To survive on this strange planet you will have to be observant of how the environment and the wildlife behaviour changes depending on the time of day.

Pikuniku is an absurdly wonderful puzzle-exploration game that takes place in a strange but playful world where not everything is as happy as it seems. Help peculiar characters overcome struggles, uncover a deep state conspiracy, and start a fun little revolution in this delightful dystopian adventure!

2D platform game with classic platform gameplay, deadly traps, dark storyline and unique hero abilities, all in Tim Burton's-like audio-visual.

Retro-inspired tower defense and action platformer SolSeraph reinvents the classics! As the guardian of humanity, civilization is in your hands. Build your cities and set up defensive structures to protect them from the constant threat of monsters, then descend into their lairs with sword and spells to eliminate them for good. Explore floating islands, ancient caverns, and lost cities. The world is yours! Brought to you by ACE Team, the creative minds behind Rock of Ages and Zeno Clash, SolSeraph combines action and strategy for a rich and unique gameplay experience reminiscent of the 16-bit era.

Semblance is an innovative platformer with deformable terrain, set in a beautiful minimalist world. It’s a game that asks, what if you could deform and reshape the world itself? Semblance takes the idea of a ‘platform’ in a platformer and turns it on its head.

Unbound is an atmospheric 2D Puzzle-Platformer where the main character, Soli, can conjure magic portals to travel between different realities in order to learn more about the catastrophe that ravaged his world. Inside certain portals, the physical properties of the character or world elements can change, offering new gameplay possibilities. Unbound’s artistic approach can be summarized as a dark fairy tale presented in a cartoonish style, with the purpose to give the game experience a fresh look and feel. Curiosity and exploration will have an impact on the overall atmosphere and will immerse the player even more.