Kestrel had a tiny secret, a secret no one knew about.
Ever since she was little, medicines and other substances that usually make people sleepy or dizzy didn't affect her. She wasn't sure why, but things like spices, sleep-inducing drugs, alcohol, or other kinds of drugs just didn't do anything to her.
She could get tired and fall asleep from exhaustion or collapse if she got hurt physically, but she couldn't fall asleep because of drugs, not even the most powerful ones. So, drinking alcohol was kind of like a fun challenge for her, as she could never get drunk, no matter how much she drank.
She hadn't shared this with anyone, so naturally, Ren didn't know either.
When Ren released the sleep-inducing smoke and everyone started falling down one by one, Kestrel figured it would be best for both of them if she acted like she had fallen asleep too, so she pretended to collapse.