Sonic Lives puts the real Sonic Adventure 2 cast into Infinite Lives.
You are Sonic. His real model, textures and animations are read from your own copy of Sonic Adventure 2 while you play. He runs faster and jumps higher than anyone in town, and carries items in his own hands.
Spin Dash (Q, or LT + A on a controller): roll through a crowd and knock everyone flat.
Homing Attack (E, or Attack in mid-air): lock onto the nearest person and curl into them.
Rivals: three people in your world become Shadow, Knuckles and Dr. Eggman (also read from your SA2), and they come after you with the Chaos Spear, a Hammer Punch and the Egg Cannon.
Single player. You need both Infinite Lives and Sonic Adventure 2 installed on Steam. Nothing from either game is in this download. Pick Continue (or start a career) on the title screen to play.
Credits: SA2 file formats learned from the X-Hax SA Tools and SA2B Modding Guide documentation (no code copied). Sonic Adventure 2 and its characters belong to SEGA; Infinite Lives belongs to MDickie.