Character Replacement

A character replacement, also known as an Impostor!Sue, a Replacement!Sue, or a Doppelgänger!Sue, is what you have when a character is so badly out of character that mere possession or Suefluence no longer explains their behavior. Rather, they have actually been replaced by a look-alike Sue or Stu. As Trojie explains in her Slash-Sporking Guide, there are a few indicators to suggest that a character has been replaced:

"A name-change usually denotes a replacement. As does a complete moral transplant: for instance, Harry Potter driven by guilt and ANGST to wander around killing Death Eaters and capslocking about it is probably possessed; Harry Potter killing Death Eaters for fun and profit and not angsting OR capslocking about it is mostly likely a replacement. Dark! versions of heroes and good! or conflicted! versions of villains are not necessarily replacements - the key is to read how the author has justified it. If they've genuinely been extrapolating from canonical clues, hints and behaviours, then it's probably a case of possession. If they've completely changed the character's motivations, then you're most likely dealing with a replacement."

When a character is replaced, the original is locked up in a plothole to prevent them from interfering with the fic. The duty of the agents is then to find the original and free them, and to charge and dispose of the impostor as they would a regular Sue (not necessarily in that order). Impersonating a canon character is added as a charge.

Examples

 * TOS, Mission 22: To Know Where You Are Going, featuring Replacement!Arwen.
 * RC #999, Mission 1: Family Ties, featuring a Snape doppelgänger, Sithchean Bran.
 * Numerous characters in That Series, Cregga and Mariel most obviously.

A problem arises with Stus that try to impersonate Captain Jack Harkness. Jack can't stay dead, and for some reason his impostors have this characteristic, too. On their one encounter with an Impostor!Jack, Agents Emma and Tasmin decided to dispose of the impostor by taking him into Greek mythology and chaining him up next to Tantalos.