Thanks Eeveecraft! Your answer help a lot. The voice indeed doesn't have a form now, but when I was a kid I often dreamed about a shadow by my bed, maybe that was its true form. And it does voice its own opinions that are different from mine and sometimes it tell me about random things in the past which I never thought I still remember.
I have never created a tulpa before but I always have some kind of voice in my head since I was a kid. It sounds like my voice but it always say the opposite to me, for example if I got angry it will try to calm me down, or when I'm doing something it will give me its opinion. What is that voice, is it a tulpa or it's something else? ( sorry about my bad English :(( )