|The Fool in The Tarot|
The "man child" trope has a derisive name, but I don't think it's automatically a bad character within the realm of fiction. I go to books and movies because I want a satisfying story and stories usually need a character to learn something. With this trope, we tend to start with someone naive, which could mean anything from emotionally stunted to frighteningly optimistic, who needs to be kinder, better, or more self-aware. While rarer in popular entertainment, this "man child" has shown up as a female character (see: Amy Poehler in Baby Mama , Cameron Diaz in Bad Teacher ). So if we're talking more classic, gender-neutral archetypes, this person would be closely related to The Fool, someone usually more clever than smart and unable to see the big picture.
I'm most interested when this character a) is a fictional entity with whom I don't have to interact with in reality if I choose not to and b) does immature things that have consequences.
Frank (Rainn Wilson) in Super (2010) gets to don a mask, defeat the bad guys, and save the girl, but he has to shed a great deal of blood in the process. Jimmy in James Gunn's first and only novel, "The Toy Collector" (2000), focuses on drugs and his collection of toy robots to the exclusion of family and friends. Peter Quill (Chris Pratt) in Guardians of the Galaxy (2014) can play Han Solo as long as he wants, but he has to fight to put together a family that resembles the love and support he has before leaving Earth as a child. The through line of all these works is both the celebration and deconstruction of the eternal childhood. Director/writer James Gunn gives us characters who live the power fantasy problematized by real world outcomes.