Alex Krychek

At first, the young Alex Krychek seemed eager to help Agent Mulder with his work. However as time progress, Krychek was revealed as a double agent working under the Cancer Man.He not only help thwart Mulder's attempt to reopen the x-files, he also had a hand in the abduction of Agent Scully. Krychek also attempted to kill Mulder when Mulder tried to rescue Scully. No trace of Krychek was found after the disappearance of Scully.

The "ratboy"(Krychek's nickname on the internet) reappeared when he killed William Mulder at his house and framed Mulder for the murder. The enraged and drugged Mulder almost killed Krychek afterwards, Scully barely stopped him by shooting Mulder in the shoulder. The ratboy escaped, only to accidently assasinate Melissa Scully(Dana's sister), during the attempt on Agent Scully's life. Cancer Man ordered for his exercution after Krychek's mistake. He narrowly escape, but he is now in possession of the ever elusive MJ disk.

The ratboy turned up several times after that. More recently, he provided Mulder with tips that led to the arrest of a militia group. Then he and Mulder head off to Russia in pursuit of a virus(actually more like black alien goo)named the Black Cancer. They were captured, but using his knowledge of the Russian Language(probably his native language), Krychek befriended their captors, while Mulder was trapped and experimented on with the Black Cancer. Mulder later broke free, bringing Krychek with him and leaving him in the forest, where Krychek encountered some men who sawed off his arm to "protect" him from the experiments"No arm, no test!" It later turned out that Krychek might have been behind this along, and he seemed to be a high level KGB or some other Russian agency operative.