Katherine forces Matt Donovan to help her mimic Elena’s mannerisms and history so she can go undetected.
Katherine, surviving inside Elena’s mind, works with her daughter and a Traveler witch named Mia to make the possession permanent. [S5E12] The Devil Inside
Caroline Forbes admits to "Elena" (Katherine) that she slept with Klaus . Katherine forces Matt Donovan to help her mimic
Believing he is speaking to Elena, tries to reconcile, admitting he needs her "goodness" to stay in the light. surviving inside Elena’s mind
Katherine (as Elena) coldly dumps him, telling him they are over for good.