What is a consigliere, and where does he fit in?
See the scene at the wedding when Michael and Kay are eating their lasagna.
MICHAEL: "Not a Sicilian, but -- I think he's gonna be consiglieri."
KAY: "What's that?"
MICHAEL: "That's um, like a counselor -- an advisor -- very important to the family."
The consiglieri is the only one that takes orders directly from The Don, which is why the position is so important. And that's why it was strange Vito would have a non-sicilian as his consiglieri. Sicilians have a strong belief in "omerta", the code of silence. Where as Hagen was an adopted kraut, mick.