I find amusement in thinking of characters who would make everyones' lives hell if they showed up.

Koishi Komeiji (as a student). Isn't human so can't be killed by a Death Note, has no presence so can't be noticed if she doesn't want to be and cannot be read by Cass, can see into and manipulate the subconscious so can tell if Light or Lelouch is controlling someone... And also acts entirely on whim. Harming her would invoke the wrath of her sister, a mind-reader with a nuclear arsenal living in a realm which cannot be entered by non-native supernatural beings without causing an interdimensional incident.