If you were wondering if you could at some point play as Kaine in Nier Replicant, you will find the answer in this guide, but be warned the following below will contain Major Story Spoilers.
Can I play as Kaine in Nier Replicant?
In short, players are able to play as Kaine after they have seen four of the first endings, including Ending D. That said, if you want to play as Kaine, you will need to create a new game after you completed the part where you need erase yourself from existence in order to save Kaine. At this point, you will need to create a new game and then play to the point where you are fighting the second boss in Aerie.
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After this fight, the game will put you in control of Kaine, and you will get to play her for the rest of the area. However, this will be the only part you will get to play as her, and there isn't an option where you can play as her for the rest of the game.
Nonetheless, this doesn't mean you can't have fun playing as her, as she has some quire power abilities that can wreak havoc on shades. Along with this, she is super fast, and both her abilities and movement speed seem to make up for her lack of any magic spells.
For more on Nier Replicant, we at PGG highly recommend checking out How to increase FPS on NieR Replicant on PC