In the present day, the first time Katherine Pierce was introduced was when she was impersonating Katherine succeeded in a deviously brilliant scheme, using Traveler magic to become a enger in Elena's head and later take over her body completely.
Josephine Lightbourne was always human, but she could continue to live on through different bodies because of scientific accomplishments. After killing his daughter and several other innocent people during the Red Sun, Russell made it his mission to regain his family. Eventually, he succeeded, returning Josephine to the world of the living using another girl's body. Such an invention allowed Josephine, along with the rest of the Primes, to live on. But, in season six, Russell put his daughter inside Clarke Griffin's body. Such a choice created tension, conflict, and a mission to save Clarke.
Katherine: More Practice Impersonating Elena
Way before Katherine even considered jumping into Elena's body, she had been pretending to be her Doppelgänger. The first time in the current day that Katherine is seen, she is pretending to be Elena. Katherine easily convinces Damon and John Gilbert that she is Elena.
Afterward, Katherine occasionally impersonated Elena, sometimes of her own wishes and others because it was asked of her, such as during Damon's attempt to kill Klaus. With all the times Katherine had pretended to be Elena, having to literally be Elena was not as difficult.
Josephine: Her Purpose
Josephine was brought back to life by her father and placed in Clarke Griffin's body. When she took over Clarke's life, Russell told Josephine to blend in as Clarke and learn how many Nightbloods arrived on Sanctum with the new arrivals. This gives Josephine an initiative when she walks around pretending to be Clarke, rather than immediately revealing she is really Josephine.
By being Clarke, Clarke's friends and family were more likely to be open about the truth given they knew Clarke already knew such details, such as that Madi had Nightblood, and they knew how to create synthetic Nightblood.
Katherine: Did Research
Nadia suggests that since Katherine is Elena now, Katherine needs to know everything about Elena, especially since her impersonations are not meant to be in short bursts but long-lasting. With that in mind, Katherine and Nadia used Matt, Elena's human and compellable best friend.
Without vervain, Nadia and Katherine compelled any information they needed about Elena, her life, and her friends. By doing so, it was a better guarantee that Katherine could pull off Elena Gilbert's identity.
Josephine: Previous Experience Body Snatching
Clarke was not Josephine's first host body. For over two hundred years, Josephine experienced life through numerous sets of eyes. This was not a new encounter for Josephine, and instead, she continues to act like her life didn't stop.
Josephine gets revenge on the friend that had killed her last body, and the way her existence is written makes it clear that all the Primes have history and acknowledge each other as family. Such history also highlights Josephine's past with Gabriel.
Katherine: Survivor's Move
Katherine was a survivor, and nothing was going to kill her. As Katherine lay on her death bed, dying from old age after ingesting and later losing the cure, she had one plan in mind. Katherine and Nadia hatched a plan to continue Katherine's life by using her Traveler blood to become a enger in someone else's body and later take over.
Katherine's unlucky person to take over was Elena and made her move in the final moments of her life.
Josephine: Struggled To Be Clarke
One of the elements that made Josephine embodying Clarke an entertaining arc was that she was actually horrible at it. With no true memories of Clarke or understanding who she had been, Josephine is left with a brief grasp of who Russell had gotten to know and the names and faces of Clarke's friends. Otherwise, Josephine makes absolutely every decision Clarke wouldn't, including letting Madi go to school, writing with the wrong hand, and using the phrase "chill out."
Josephine's impersonation was an intriguing attempt and definitely amusing to watch. However, it was not a surprise that Bellamy could tell something was wrong with Clarke and figured it out quickly. Even with Murphy's help later, Josephine's impersonation was better but not great.
Katherine: Elena's Friends Fell For It Longer
Katherine pulled off existing in Elena's body for three weeks before Caroline and Stefan figured it out. For those weeks, Katherine had successfully convinced everyone she was Elena.
While the group may have noticed things were different about Elena, they may have chalked it up to the breakup with Damon. Everyone believed Katherine was dead, and they had no reason to consider that Katherine had one last master plan as she died.
Josephine: The Inside Look Into Clarke's Head
The 100 gave a look into Clarke trapped within her mind while Josephine took possession. By doing so, viewers got a look into Clarke and her realizing what had happened. However, it also allowed for a unique relationship between Clarke and Josephine. They both wanted control of Clarke's body and were willing to kill each other to get it.
However, the two interacting also gave Josephine a look into Clarke's memories and a chance to understand her more. As Clarke's body was dying, Clarke and Josephine removed Josephine's memories from Clarke's head, which gave Clarke a chance to understand Josephine and her past better.
Katherine: Hurting Many People's Lives
When Katherine died, she didn't go easily. Knowing she was giving herself up when she arrived to say goodbye to Nadia, Katherine was aware of how the night would likely end. However, it is notable to say that Katherine proved to cause chaos before leaving Elena's body.
Her first act as Elena was breaking up with Damon so Katherine could go after Stefan. Although shocked to hear Caroline slept with Klaus, Katherine used it to her advantage when she repeated it loudly, knowing Tyler would hear. Lastly, one of her most diabolical acts toward Elena had Katherine inject Elena's body with the Ripper Virus so that Elena would desire vampire blood.
Josephine: It Wasn't Personal
One of the biggest differences between Katherine and Josephine was that Katherine was deeply jealous of Elena. Elena had everything Katherine wanted, so Katherine stole that life for herself when the opportunity presented itself. It was just another thing Katherine did in her antagonistic relationship with Elena.
Previous to taking over Clarke's body, Josephine had never met Clarke or any of Clarke's friends. Josephine hadn't even chosen the host body. This was not a revenge plot against Clarke and her friends. Josephine did find amusement in messing with Bellamy and Murphy, and Murphy did help Josephine with her Clarke impression. However, none of this was meant to be a personal attack. Josephine wasn't trying to ruin everyone else's lives by starting her life again. Josephine has no emotional connection to anyone, allowing her to feel freer in manipulating those around her without previous personal ties.