My understanding is that it is usually the number three position in the US groups (formally speaking). But the particular powers and responsibilities depend on the family.

Sometimes it's an older guy; sometimes not. Usually the counselor doesn't have line responsibilities (crews reporting directly to him) but this isn't always the case either. Some counselors want to remain hands on. There are examples of counselors directing hit teams and so forth and so on.

There are some people who have acted as the stereotypical wise advisor/fixer (Murray Humphries). There were other guys who were semi-retired and only gave suggestions to the putative boss but the counselors have so much power that their "suggestions" were often really orders (Accardo in Chicago was a good example of this type).

Many times the counselor may get a piece of whatever the boss and underboss get. But the counselor is not the person who speaks for the boss in his absence-that's usually the underboss. And for the most part there haven't been too many counselors (Caponigro was an exception) who became bosses upon a boss' untimely demise from a sudden case of death or upon permanent incarceration. Many times that is a captain, an underboss or a driver/bodyguard.

The original idea back in the 1931 structure creation was that a consigliere would be someone who knew the family inside and out, knew the rules, could settle minor issues that the boss didn't want to deal with, occasionally represent the Family in meetings/disputes and even provide appeal to soldiers within the Family if the boss made a bad decision. This last was pure malarkey of course as any consigliere who wanted to keep breathing never disagreed with the boss.