According to our very own Suikosource
, it's not a True Rune because Windy can only wield two at once.
I'm confused, though, as to whether Windy herself had a Conqueror Rune or if she simply gave them to Kwanda and Milich. If she herself possessed the rune, wouldn't that mean that she, too, would be controlled by it? Or maybe she was, and that added to her grudge against the world to make her do evil things? Or maybe the black runes given to Milich and Kwanda are simply "sub-runes" of the Conqueror Rune.
One could study the world of Suikoden for years. I don't think anything on the page mentioned that Windy didn't possess the rune, though.