My copy of 'The Nameless Monster' is actually tucked away in a box somewhere from my last move, so the details are a little hazy, but I’ll try to piece it together. It’s from the picture book the Monster reads in 'The Book of the Nameless Monster', right? The one within the 'Monster' manga by Naoki Urasawa.
From what I recall, the story in that book is about a monster who devours people and takes their names and faces, but in doing so, becomes more and more lost. It forgets its own name and wanders, hollow. I always read it as this perfect metaphor for Johan in 'Monster'—he’s consumed so many identities and lives that he’s become this void, this 'thing' with no core self left. The nameless monster isn’t a separate character; it’s the essence of what Johan became, or maybe what he always was.
It’s less a 'who' and more a 'what'—a parable for the emptiness at the center of the story’s real monster.