Following an explosion after his defeat at the hands of Kasumi, what was left of his body was soon found by Victor Donovan. He was reassembled and outfitted with cybernetic parts. The process of mechanization erased Raidou's memories however, leaving only his desire for power and destruction.
Raidou, we haven't forgotten your inclusion in Last Round. Here's the Raidou general discussion thread, since the forum itself is a bit bare.
"I want your skills! Show me your secrets!"