I'm looking for a manga where the protagonist seems to be cheerful and childish, but hides his sadistic and much darker (and cooler) side. And the story is about him outwitting his opponents who underestimates him, which may include a larger darker plot. Preferably from the shounen genre.

oh, definitely Death Note. it fits pretty much perfectly.

Oh, the protagonist must be a cute, childish and cheerful outwardly. I think the correct definition on this type of character is yandere, which is the opposite of tsundere. I don't think Light fits the bill.

You're talking about someone more like Hunny-senpai's dark side in Ouran. Only it's not a main theme there; there's only a little part of his nature that you see. (And that happens to be shoujo.)

Nothing springs to mind off of the top of my head. I know there are threads about yandere here and here.

If you've ever thought or said "Nice Guys finish last" and really meant it, then you should probably read this LJ post by DivaLion. It's incredibly insightful whether you're male or female.

From a bumper sticker I like:
"If you're gonna ride my ass, at least pull my hair."

Death Note isn't even a romance manga... Considering that Yandere and Tsundere are terms usually used to describe a person's attitude in a relationship. Kind of like how Yaoi has seme and uke...

So yeah, the two criteria before you recommend:

1) The guy has to be in a relationship, so it would most likely be a romance manga of some sort.

2) Actually correctly identifying the Yandere.

That being sad there's a real low percentage of Yandere. And nothing really comes to mind, especially in terms of guys.

Here's an informational pic, anyways.

source: animenewsnetwork

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Pheromomania Syndrome the guy that looks like a girl
Ai wo Utau Yori Ore ni Oborero! the guy that looks like a girl

Feed your head.

the demon guy from majin tantei nougami neuro is like a total ditz, but he is a demon and has a much darker side

hi there here my advice for your needs besides Death note

Lost Brain - a death note wannabe next hit

Doubt - all the caracters are cheerful but they hide some interesting secrets. Think about mixture of the wolf game (a child game) and the SAW.

You should try Akumetsu .
One of the best mangas that I am reading.

Quote from Liria

You're talking about someone more like Hunny-senpai's dark side in Ouran. Only it's not a main theme there; there's only a little part of his nature that you see. (And that happens to be shoujo.)

Yeah, you're right, the protagonist that I'm looking for is something like a much, much darker Hunny. I guess it doesn't have to be shounen, but even if it's shoujo, I want the story to focus more on the contrast between his cheerful outward appearance and his true dark nature rather than realtionships alone. Sort of like a manga version of Dexter.

Quote from Gokuthegrate

You should try Akumetsu .
One of the best mangas that I am reading.

I totally second this! 😀

I'd also suggest this thread: http://www.mangaupdates.com/showtopic.php?tid=6247

[url=http:////www.mangaupdates.com/showtopic.php?tid=42718][img]http://i366.photobucket.com/albums/oo105/CatzCradle64/Avis-Sigs/FunghiPower_zps2a00ba56copy_zpsfc03990e.png[/img][/url]

Quote from CatzCradle

Quote from Gokuthegrate

You should try Akumetsu .
One of the best mangas that I am reading.

I totally second this! 😀

I'd also suggest this thread: http://www.mangaupdates.com/showtopic.php?tid=6247

That works.