The Terror of the Thames.
When I was just a wee thespian with a Cats obsession, this character was sort of mysterious to me. I had the poem, his song on the Broadway cast recording, and a tiny colour photo of him and Griddlebone in a book about the broadway production. I knew he existed. But he wasn't on the filmed version, making him an elusive character.
Mungojerrie and Rumpleteazer also caused me similar distress but I only just learned what their deal was in the OBC like yesterday.