There are a few of these traits in Kamina in DOUBLE K. Quoth captainosaka:
i like how kamina's plan seems to suggest that he's regressing back to the mental state of a vindictive third grader under all this duress though you could probably argue that he never grew out of it to begin with
Clubs Deuce/Courtyard Droll from Homestuck has an extremely childish view of the world that borders on Ambiguous Disorder. Word of God says he basically thinks of his fellow gangsters as his goofy family, apparently being completely unaware of what a gangster is (granted the Midnight Crew are kind of losers).
C. S. Lewis, in a similar spirit, once said "When I became a man I put away childish things, including the fear of childishness and the desire to be very grown up".
Amical from morphE. He may be the head of a business with a mansion full of guards. He may be intimidating when he's waving a gun around and forcing people to fight to the death. But if you call him out on being vile or don't let him get his way then he'll start crying and whining that everyone is mean to him and that he works so hard to please and gets no respect. It's as childish as it is manipulative.
David Walkertoninvokes this trope in order to avoid responsibility (and also partially due to a traumatic childhood event) and can actually be mature and insightful when the situation calls for it. However, even when the need for the facade ends along with the main plot, he continues to act the part, so it's also a legitimate part of his personality as well.
Joyce qualifies as well; though she's generally more responsible than him she shares his fascination with cartoons, burps, and fast food as well as his goofy demeanor. It's part of what makes them so perfect for each other.
Not to mention Robin, especially later in Shortpacked!