Sonic was just relaxing on the grass of Green Hill Zone while Tails was working on his Gadget's when suddenly Silver Approached them. Silver told Sonic & Tails that Blaze was fighting some person named "Emperor Kai" and then she suddenly started acting weird. Sonic suggested that they go to knuckles to have him assist them, and they did. But right when they approached knuckles, Kai Blaze and Kai Vector appeared. Silver kept saying "Blaze, it's us! Don't you remember us?!", Knuckles said the same thing to Vector. But all they did was respond with: "We're no longer Mobians, we're part of the Kai Army, we are Kai Blaze & Kai Vector." At that point there's a boss fight between Sonic, Knuckles, & Silver vs. Kai Blaze & Kai Vector. Meanwhile, it turns out the Gadget Tails was working on was something that can "Disinfect" other people, but they had to be knocked out. So after the boss fight, Tails uses the Disinfecting Gadget and Blaze & Vector wake up and wonder what's going on. They tell them the whole story, at this point they become Assist Characters. Assist Characters are Characters used to help in Boss Fights when your Health is really low.
Boss 1: Kai Blaze & Kai Vector
Playable Characters: Sonic, Knuckles, & Silver.
Support Characters: None.