Despite his dark and foreboding appearance, Sinyster is a gentle wolf at heart, but it takes a lot of time and effort to get past his defensive emotional barriers. He can easily live up to his name when angered or threatened, but otherwise stays calm and silent. Sinyster generally avoids contact with others unless absolutely necessary and stays close to those he trusts, which, again, is hard to gain.
Best rule to keep in mind around him is this: respect him, and he'll respect you. Fail to do so could cost you dearly.