Nothing

Nothing is a huge, apparently human, male. He first appears in Aftermath.

"Aftermath"
Nothing waits for Karrin Murphy sitting on a swing in Buttercup Park. Apparently unbothered by the October chill, he's at least 7'4" tall, heavy muscles, an athlete's carriage sled-dog blue-white eyes, and a basso voice. When Murphy joins him, he politely bow from the waist.

When Murphy comments he's not human, he admits he no longer is; he is in fact gilled.