- Corpse Party has Yoshikazu Yanagihori, former Cool Teacher. As his mental illness progressed, he reverted to a childlike mental state, and eventually couldn't even convey his thoughts. People noted that he was still the same person he was before, so it was shocking when he apparently abducted four children and brutally killed three of them. He didn't actually kill the children. He was manipulated into kidnapping them, but he was just an accomplice to the real killer... the surviving child, who was actually a malevolent spirit.
- King Erik from Fantasy Life not only acts like a child but looks like one despite having a middle-aged wife and an at least teen-aged daughter. It turns out he's in a magic-based prank war with the equally man-childish Former Dark Sultan of Al Majiik and that one of the spells turned him into a child.
- Yellow Heart, from Hyperdimension Neptunia Victory. Despite being physically powerful and having the visible age of the other CPUs (at least), she does everything to please her "parents", can only read and write very simple words, and is generally sharp as a sack of particularly dull hammers. Turns out she's an artificially altered Peashy, and therefore actually a child, despite "the big me" looking like an adult.
- Tingle from The Legend of Zelda series. A 35 year old man who believes he is the reincarnation of a fairy and spends his spare time floating around on a balloon searching for other fairies. Ramped up in The Legend of Zelda: The Wind Waker, where his HQ is a tower where his cohorts spin the top around to make magic happen... or something.
- Mae from Night in the Woods is a twenty year old college drop-out who acts like a rebellious teenager, compulsively committing petty crimes and having a hard time talking to other people her age about more adult concerns. This strains her relationships with her parents and most of her friends, except her partner in crimes, Gregg.
- Super Mario Bros.:
- Mario easily fits into this category, though he's a distinctly positive portrayal of it. Aside from being a never-ending fountain of bubbly joy, he has absolutely zero qualms with playing with the Mini Marios his own company manufactures, indulges himself in any kind of fun activity regardless of how juvenile, and seems perpetually impossible to take the life-threatening obstacles he overcomes with anything but a whoop of excitement and charging headlong at it.
- Mario might be full of youthful exuberance, but Luigi often just comes across as childish and immature. It is particularly evident in the Mario & Luigi series, where he's prone to crying like a child, though latter games in the series tone it down some somewhat. Superstar Saga and Partners in Time has loud, exaggerated sobbing, Paper Jam has him quietly crying into his hands. Even his voice comes across as more childish than Mario's, despite Mario having the more higher pitched voice of the two.
- Bowser shows obvious signs of immaturity on a regular basis; he has nasty mood swings that scare even his most loyal servants, he never feels responsible for his failures, always looking for someone else to blame, and he is never satisfied with what he already has (he has castles in almost every land in the world), always wanting more. It gets even funnier when he meets his baby self in Partners in Time: barely anything differentiates them in terms of personality, which clearly shows that Bowser has barely evolved mentally since his infanthood. No wonder why Kamek lacks patience...
- Justified in the case of Grunt from Mass Effect. Since he didn't technically HAVE a childhood (unless you count accelerated growth in a vat while a Mad Scientist plays context-free videos for you to be a "childhood"), he comes out of the tank nearly fully grown but without a great deal of comprehension of the world beyond "if you squeeze this thing here, the gun goes bang and kills something". As a result, while he's fairly smart, he still plays with action figures, pulls out Buffy Speak on a regular basis, and mostly views Shepard, the player character, as a Parental Substitute who will teach him how to do all the important krogan things like break stuff and set it on fire. In the Citadel DLC, after he sets a Citadel Security car on fire, steals it and crashes it, this exchange can happen:
Shepard: Grunt, apologize to the nice man for setting his car on fire.
Grunt: (looking at his feet) I'm sorry I set your car on fire...
[notices Shepard's expression]
Grunt: ...and I won't do it again.
[...and as soon as the police have left]
Shepard: I love you, Grunt.
- Metal Gear:
- Big Boss, of all people, in Metal Gear Solid 3 and Metal Gear Solid: Peace Walker. He has a Phobia of vampires, he gets ridiculously excited about eating things, he makes childish noises of disappointment when told not to eat things, is rather fixated on his mother figure, thinks making cardboard tanks and using bananas as firearms is a pretty neat idea, gets along as equals with a literal child, and still believes in Father Christmas.
- Otacon in his first appearance in Metal Gear Solid comes across this way. He's an intelligent but naive tender-hearted dork who likes dogs, robots, and anime and just wants to build cool stuff to protect people, and he is way out of his depth in a cast full of conspirators and killers.
- One interpretation of N's character in Pokémon Black and White. It's not played for laughs. Indeed, it may actually be Foreshadowing of his... father issues.
- Heck, N's upbringing in general does a lot to explain his Manchild tendencies, especially when you see his toy room.
- Sonic the Hedgehog games:
- Doctor Ivo "Eggman" Robotnik has shown some shades of this. In spite of his advanced knowledge of science and robotics, Eggman also appears to be very immature and is prone to throwing childish outbursts whenever something doesn't go his way. His television and comic depictions also exhibit some of these traits, most notably his Adventures of Sonic the Hedgehog counterpart.
- Vector the Crocodile is also an example. Despite being 20 years old, he's no more mature than the rest of Sonic's gang, often arguing with 6-year-old Charmy Bee.
- F.A.N.G. from Street Fighter. He prances around and gleefully follows Bison's orders like a child and whenever his plan go wrong he is prone to throwing temper tantrums. It's safe to say his other fellow Shadaloo lackeys have little patience for his shenanigans.
- Luke fon Fabre from Tales of the Abyss starts out very gullible, emotional, demanding, self-centered, petulant, frustrating and stubborn. Most of the time, he acts like he's seven years old. Which makes sense, because technically, he is. He was cloned from the real Luke fon Fabre seven years prior to the game's opening, and retained none of the original Luke's memories.
- Max Hass from Wolfenstein: The New Order is an almost literal version, due (presumably) to the fact that a good 1/4 of his skull is missing. His room is filled with toys (some of which you can go out of your way to recover for him at the base), and he's got an acute fear of loud noises (namely buzz saws).