Mello... yes your right. He thinks ahead. Has very unique traits. Makes him an N. He let his emotions get to him which makes him an F. My apologies. I don't really know much about mello as I can't really remember much about him from the show. (watched the show when it first came out)
However, Ryuk strikes me more as an Introvert. In the shinigami world, he is more of a loner and stays by himself. Does things on his own. Although with light he just hangs arounds him to see what will happen. He has intuitive traits although I'd dub him more as a sensor as he devises situations by what he sees and not really by his thoughts.