Assist is a move that appears exclusively in the multiplayer mode, Eggman's Sonic Simulator, in the Wii version of Sonic Colors. It allows one player to stand by while the other traverses the level on his/her own.
When a player presses the 2/Y/A button (depending on the controller) in one-player mode in the Sonic Simulator, the Virtual Hedgehog/Mii who is controlled by that player turns into an orb. This hovering orb follows the other player, who can now continue alone. Although the player is powerless to control the orb, his/her Virtual Hedgehog or Mii is invulnerable. At any time during the level, the player can revert into Virtual Hedgehog. When switching between the Virtual Hedgehog and Mii, the player does gain some height which can be useful for some purposes.