Mr. Grumpy is a fictional character known for his sour demeanor and constant frowning. He is part of the popular Mr. Men book series created by Roger Hargreaves. Mr. Grumpy often finds himself annoyed by the cheerful antics of others, which adds to his grumpy personality. Despite his negative attitude, Mr. Grumpy's character serves as a humorous reminder of how not everyone is cheerful all the time. His interactions with other characters, like Mr. Happy and Little Miss Sunshine, highlight the contrast between different personalities in the series.