Edit
Mr Happy has quite a simplistic story, as do most of the other characters. It involved him discovering Mr. Miserable (who looks exactly like him) and helping him become happy. Mr Miserable becomes very happy at the end of the story so Mr. Miserable moves next door to Mr. Happy. Roger tells the reader to turn the sides up of your mouth at the end. In 2008, Mr. Miserable is not seen, but since it is Dillydale, you can see Mr. Grumpy's house instead.
Mr. Happy in the 2008 TV series The Mr. Men Show looked exactly as he did in the books. His limbs are slightly smaller, he has a less circular body and occasionally, visible eyebrows. He co-hosts a morning show called "Good Morning Dillydale" with Little Miss Sunshine. His catchphrase is "Wonderful!" because he keeps smiling and never gets sad. In the US version, he has a deepish voice, sounding like a stereotyped game-show host. In the US version, he is voiced by Sam Gold. In the UK version, he is voiced by Simon Greenall in season 1, and Rob Rackstraw in season 2.