Have a idea for a twist on a game but I think it's going to be really obvious and I'm unsure if I should try it. Any advice?

I've been thinking on an idea for a small campaign (4-5 sessions max) where getting closer to the end the group will have to fight a mentally controlled friendly NPC by the villain of the story. Idea is that this NPC is someone without any memories of who she was before waking up one day being found by the rebel group she now works with alongside the PCs, her backstory at the end being revealed that she was a creation by the BBEG from the very start, allowed to have her own life but mentally controlled in small ways without her knowing till the BBEG decides to use the full mental control and use her unleashed powers.

This is kinda my problem with this, I feel my players will be smart enough to realize something wrong with her from the start as someone without any knowledge of their past will cause some red flags to appear. I also think that the twist could be considered too dark cause the idea at the end is that she will die even if the PCs try to save her the best they can after the BBEG will use a kill switch, mostly as a way to gave more weight for the final fight against said BBEG at the end of the story.

Any advice on how to make this work? Any idea is welcomed.