The fading metaphor is pretty powerful. I felt like maybe there were too many times I had to press space to get to the final invisible state... Also, I'm thinking maybe you should have the text fade away automatically as well, to reinforce the fadedness. And that would make it so that the very last utterance also fades into nothing.
Overall, it's really interesting to me what kinds of emotions and empathy you can elicit through very simple concepts and minimal design. I wonder, though, whether anyone who doesn't want to read text is going to get anything out of this. and whether anyone who is okay with reading text is already someone who tends towards compassion and empathy... Any thoughts on that?