Conversation so far:
Was Mewtwo in a book?
Mewtwo has appeared as a central character in several books related to the Pokémon franchise.
What was the first book Mewtwo was in?
Mewtwo Strikes Back
What was the next book Mewtwo was in?
Mewtwo Returns

Question: Was he a popular character
Self-contained rewrite: Was Mewtwo a popular character?