Sonic Battle was believed to be Sonic's first step into the fighting genre; however, it was not. There was also an arcade game called "Sonic: the Fighters", which is extremely rare in present day.
Sonic Battle takes the Sonic Adventure characters into this game, but it also adds a new character to the game, named Emerl later on as the story goes. Emerl can copy the moves of it's opponents using "Skill Points" it attained by battling.
1. http://www.planetnintendo.com/sonicbattle/ - Sonic Battle X.
Developer: Sega (not Sonic Team)
Platform: Game Boy Advance
ESRB Rating: Everyone