Someone told me that there is a character in Dota that can be only unlocked with cheats and is extremely good Is it TRUE because i dont trust him?

You must be thinking of an old version, in which the secret character was the Invoker. He wasn't particularly good at the time, just interesting due to the nature of his spells and the use of the ball combos for different spells. He is now a normal character though. In 6.59D you can unlock the Tauren Chieftan. He is rumored to be the new hero for 6.60. The unlock process is pretty complicated. I woul d suggest you you tube it, there are videos explaining how to unlock him.