Ashura

From Sonic - Wiki on Neoseeker

Ashura is a glitch in Sonic The Hedgehog 2, named after a Hindu god with similar colours. Ashura appears as a palette glitch in Debug Mode in Emerald Hill Zone. The way to obtain him is to select the top of a waterfall in debug, and move Right holding C, until you pick up speed. Add waterfalls while going back and forth the same area- when your life icon changes colour, this mean the glitch has worked.

It is worth noting that Ashura is not canon; he is not even considered a true Sonic character.