Cheerful Child

Anime & Manga

 * Rimone from Simoun remains mostly cheerful, despite the burden of piloting a weapon of mass destruction.
 * Yutaka from Lucky Star is also overaged, but she can easily pass off as one, especially consider her height.

Western Animation

 * T.J. Detweiler from Recess, although like Arnold, more in the sense of being calm and optimistic.
 * Gene from Bob's Burgers is a Cloudcuckoolander with a childlike enthusiasm to tends to act before thinking.
 * Mabel from Gravity Falls. Rare are the moments when she doesn't have a smile on her face.
 * Steven Universe is quite an energetic Kid Hero. He is prone to Tender Fears, and in later episodes starts hiding his worries, but is overall happy-go-lucky and optimistic.
 * The Powerpuff Girls has Bubbles. She is even described as "the joy and the laughter" in the end credits theme song.