I'm guessing that she was just a hallucination of his after he noticed a flower bloomed near him? I mean, maybe she was actually a fairy and the flower was what happened when she died, or maybe she was an actual person and her body is actually there rotting away, but all we the readers see is the flower. Hard to say for certain how layered this really is, but the simplest answer is that she was just a flower.
@lostsage I'm assuming that there is a fixed distance that one has to be to start withering, and he just wasn't close enough to her yet. However, after he took notice of the flower he inched a bit closer and then she started to wither, probably as he reached out to her. Take notice of how he pulls his left arm away after she withers.
Could also be that the withering is instead aging, like she literally grew from a seed, blossomed, and then withered within seconds because that is his power. Rather than simply siphon the life from living things around him, perhaps he siphons their time.
Edit: In fact, now that I reread it I think the aging idea is highly likely. At first, when she appeared she was curious of why he was chained up and who he is. Children are often quite curious, and due to their innocence they wouldn't think twice about asking someone who is chained up why they were in that situation, whereas an adult would take a hint and avoid them, assuming they were a criminal or something. Notice how
wide-eyed she is, full of curiosity. Then after that, she asks him if he thinks she is pretty and if he likes her, something a teen or young adult would want to know from someone they are interested in. She's blushing a little and beaming with happiness and laughter, her
mature eyes full of love and yearning as she gazes at him. Finally after this, she asks if he'd be sad if she were to die, and then follows with her asking him to remember her, much like how the elderly on their deathbed would want to be remembered by the people they love. Her
weary eyes are somber and look a bit tired.
It's subtle, but even though her body is the same throughout the entire ordeal her eyes change significantly, from that of a wide-eyed child, to that of a mature adult, and then finally to the aged eyes of someone nearing the end after a long life. And yet, her time was so short lived.