I think mine would be “WTF” or “Grue” or something like that. Invisible basically 24/7, silent, and instantly destroy anything hit, often reducing things into pink mist. Only way you know my char exists is the mobile giant metal mountain they use as a base.
One day, city full of zombies. Go to bed, wake up, no zombies left in sight. Find a zombie somewhere, suddenly it just explodes in complete silence and nothing is left but a rain of blood. Hear a bank alarm going off, watch the walls explode into powder with no cause visible.