In short: What's Left of Me by Kat Zhang has an entirely intriguing and well executed concept, but is lacking in world building. 
 What's Left of Me presents a world in which two souls are born into one body and in which one of those souls - the recessive soul - should naturally fade away while the dominant one takes over. But this is not what happens in Eva and Addie's case. Despite many years of therapy, Eva - the recessive soul - remains, trapped in a body she has no control over. Kat Zhang has created a fully realized, entirely fascinating concept with What's Left of Me and not only that - it's also executed exceptionally well. It is so crucial in a story like this to be able to distinguish between the two different characters without creating any confusion, and Zhang manages this impressively, creating two separate, independent voices within one body. 
 I did feel as though the world building was lacking, however. I was left with major questions surrounding how this hybrid concept works and why the government sees hybrids as bad. After a strong start, the second half of What's Left of Me is spent trying to escape an ominous and mysterious government-run hospital that claims they want to cure Eva and Addie - standard dystopian fare. I feel like this is the point where the plot starts to wane and this is probably due to the lack of world building mentioned. It was difficult to become invested in a plot in which so few explanations are given for why the government has this vendetta against hybrids and why the reader should care. 
 Overall, What's Left of Me has a incredibly intriguing and well executed concept that made it stand out among a sea of indistinguishable dystopian reads. But the second half of the novel was less inventive and less absorbing due to the lack of world building. I still very much plan on continuing on with the series, however, if only to learn more about the hybrid concept. I need answers!