In a short story I wrote in the past, I had a character named the Narrator. He introduced himself as a kind of free floating consciousness that formed as a kind of accidental echo, but completely lacking form and function beyond seeing things and trying to understand the world. In the end, he turned out to be critical to the plot, but it still didn't really work out because there was little to engage the reader to the character to the narrator beyond a witty voice.
Doing it over, I think I could make him more engaging by letting things drop about him, but long-story-short, I think this form is a very difficult proposition. A key trick I've seen would be to keep it short and make it almost rhythmic or poetic in its presentation, without becoming an outright stanza'd verse. Stephen King did that in the Stand with his presentation of how the Cap'n Trips virus spread. It was a short chapter and moved with it's own kind of separate rhythm from the rest of the story.
Sort of like montage music played behind the writing.