There aren't a lot of characters in any fictional genre that genuinely frighten me, and only five that I can think of in anime. Gluttony is probably the second most frightening fictional character I've ever come across. I'm not sure what it is...he just shakes me and makes me want to curl up in an unhappy little ball and cover my ears. Threats involving him always put me more on edge than with any other character, and...well, yeah. He just terrifies me.
Does anyone else feel similarly? Any ideas why Gluttony's so..nightmarish, I guess? What about other characters in FMA that truly frighten you, if not Gluttony?