Who is Snape?

[hermys_quill]

No: HPFGUIDX 76694

Lots of discussion happened about who actually Snape is? Most of the 
people believe in half human concept(Vampire or something else). I 
also guess almost the same thing. If you read Chapter Occlumency in 
OOTP, Snape and Sirius had a hot argument and Sirius said something 
like- Dumbledore thinks you are reformed but I do not. then Snape 
said why do you not tell him, would he believe on someone like you  - 
I do not have the book right now. What could the word "reformed" 
indicate? Any ideas?
-Hermys_Quill