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 Rather, they have actually been replaced by a look-alike Sue or Stu. When a character comes close to or reaches 100% OOC, they split into two characters--one character replacement, and one canon character. The canon falls into a plot hole; the replacement takes over the character's role. 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). Kidnapped canons can often be found in canonical prisons or other hard-to-escape places. Impersonating a canon character is added as a charge.

Examples

 * TOS, Mission 22: To Know Where You Are Going, featuring Sue!Arwen.
 * RC #999, Mission 1: Family Ties, featuring a Snape doppelgänger, Sithchean Bran.
 * Numerous characters in That Series, Cregga and Mariel most obviously.
 * Department of Technical Errors, Mission 2: The Impostor, where Captain Jack Sparrow is replaced with a wangsty "pirate" who weeps over the loss of his Will Turner.
 * Tawaki, Mission 21: Seven Eighths, where the Seventh Doctor is replaced by a doppelgänger with Captain Jack Harkness' mind.
 * Every single canon character in For Your Eyes Only.
 * Most of the characters in My Immortal.

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.