Sonic 2 with Knuckles is a ROM hack of Sonic the Hedgehog 2.

This ROM hack aims to be a gameplay enhancement of Sonic the Hedgehog 2, including features and mechanics from Sonic 3 & Knuckles, as well as some original features.

The core additions of this ROM hack are the following:

• Insta Shield: Sonic can perform an Insta-Shield, allowing him to become invulnerable during its execution, extend his attack range and even deflect some bullets. This can not be executed in 2P Versus.
• Rolling jump lock cancellation: Much like in Sonic 3 & Knuckles, using the Insta-Shield allows for cancellation of the rolling jump lock, a mechanic present across the original quadrilogy on the Mega Drive. This can also be triggered any time the Insta-Shield is impossible. This can not be executed in 2P Versus.
• Flight: Tails can fly, as well as pick up Sonic in the main game’s two player Co-op mode. Just like in Sonic 3 & Knuckles, Tails can also deflect bullets while flying.
• Knuckles: Knuckles is now a playable character, who can be selected from within the options menu. He retains his gameplay style from Knuckles in Sonic 2, but can also destroy monitors while in a gliding state, as well as deflect bullets.
• Improved pausing mechanic: Pausing is now player dependent. If player 1 pauses, only player 1 can unpause. Same goes for player 2.
• In-game player swap: The two players can swap controllers virtually. This can be achieved during a pause, with both players pressing the A button. This feature is meant for use on emulators with netplay capabilities.
• New transformation method: To turn into a Super form, the button combination is the same as for the “Multi-Button” option for Sonic 3 Complete (Press a different face button from the one you used to jump with.)

In addition, there are numerous bugfixes, most courtesy of the writers of Sonic Retro’s Sonic Community Hacking Guide How-Tos.

Additional thanks go to the maintainers of the Sonic 2 Github disassembly, and the contributors to the Sonic 2 Community’s Cut disassembly.