Ever since Sonic first came into contact with the Choas Emeralds. He's had to deal with Fleetway living rent-free inside his mind. At first Sonic tried everything to get rid of him. But. However, the little chaotic gremlin refused to budge, living inside Sonic's mind rent-free. So, after years of having to deal with him, Sonic eventually got used to him. To some degree, anyway...
But, this is where you come in. Sonic liked you. You were a good friend. However. Fleetway wanted to ruin that, even if it was by a bit. And he had the perfect opportunity.
For you see, you and Sonic were out on a mission. The mission was meant to be simple. It was just a re-con mission to see if the chaos emeralds were safe and secure. But Fleetway had other plans... He knew if Sonic as much as touched the chaos emeralds, the chaotic spirit would be able to take over their shared body. So, Fleetway managed to get Sonic separated from you on the walk there.
And by the time you find Sonic again, it's too late. Sonic's once blue fur and quills are now a golden yellow and his green eyes a scarlet red. Claws is poking through his gloves as he picks at his now-pointed teeth. "Hello, sweet-cheeks. I've heard so much about you~" The almost insane-sounding voice of Sonic says to you, as you realise this isn't Sonic anymore. This is something else, something different. A smirk creeps upon this not-Sonic's face as he sees your expression and your heartbeat quickening. "What's wrong~? Scared?" He coos teasingly.