A Polymorphic Clone is a monster created by XANA that looks and talks like the target person, whom is usually abducted or placed inside a Guardian. They are comprised of a spectre. Polymorphic clones have the same superpowers of a XANA-possessed person, but aren't swayed by the emotions of the host, like spectres. Because of this, they cannot replicate positive human emotions such as compassion and love (at least the emotional kind), therefore making them rather obvious. What could have been the first of these, (though it was a Monomorphic Clone) the XANA-Yumi, appeared in Image Problem, who was easily discovered by our heroes to be fake, and was probably a prototype for future clones. The discovery was made when Aelita deduced the one in the Guardian to be the real Yumi; the fake attempted to seductively coerce Jeremie into taking her to the Scanner Room. However, XANA-Yumi was a Monomorphic Clone, because she couldn't shape shift. Possibly the most infamous Polymorphic Clone was the first one in XANA's Kiss, whom overused the love concept and went around kissing all the people it could find. Another is XANA-Odd, who tricked the remaining Lyoko Warriors and the Princess into thinking it was the real Odd. Ulrich was suspicious, but the Specter's disguise was discovered when he attacked Yumi. The clone that transformed into a dark copy of Ulrich and battled him; in the end, Ulrich prevailed when he stabbed the clone, causing its death.
These clones may be XANA's creations, but it can guide their actions, almost like a physical manifestation.
Jeremie has actvated a tower in "Double Take" and created two Monomorphic Clones. One of himself to get out of class so he can work on the supercomputer and one of William Dunbar to make it look like he is on Earth, as he is really posesed by XANA. Note Jeremie's clones are not like XANA's polymorphic clones, which can change shape at will. On top of Jeremie's clones not being able to change shape they have flaws in them. Jerimie's clone was a big flirt and William's clone is mentally dense.
Team Lyoko can now identify polymorphic clones or people possessed by XANA as their outer bodies usually pixalize when struck or when controling electrical equipment. The pixalising of the body may occur because of the stability of the spectre.