The Ikemen engine allows for intense AI-only battles to test character strength. Roster Overview: More Than Just Sonic

Any "APK" file found on third-party sites claiming to be this game should be treated with extreme caution. They are often unoptimized ports or may contain malware.

What you are seeing online is a (a ROM hack or a standalone engine recreation) that takes the original 2003 GBA title and injects it with modern steroids.