Recommend me series that started of as lighthearted or comedic but became more serious/dark later on.

Any genre are fine, no BL/GL though (i'm not into them). If possible no harem/reverse harem too, but it's fine if it's not the main focus/doesn't play a big part in the story.

I'm okay with manga, manhwa, manhua, even novels. Please don't recommend me popular series or series that already adapted into anime. Example i could think of at the moment is Roland Roland

i know you said no anime adaptations, but the series is very unpopular/not well known: school days. it fits the request perfectly though besides the adaptation part ♡

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Didn't expect this to be recommended, especially since i said no anime. But i know that anime already, i think it's pretty well-known among people who's into anime for a quite some time. Thanks though, at least you've tried

Idk about dark...? But I've heard Negima starts as a harem but turns into a battle shounen?

Dead Dead Demon's Dededededestruction probably, though I don't think anyone who knew Asano Inio's work beforehand was surprised by the turn.

Ciel - The Last Autumn Story starts out as a pretty offbeat comedy and then turns really serious (and honestly kind of confusing).

And, while it doesn't exactly ever get dark, Helck does become more serious later on.

ONE's work (Mob Psycho 100 and One Punch Man (ONE)) also often has these qualities, though they're so famous now, that I don't think anyone is surprised to learn that they aren't just comedy.

I Married the Male Lead's Dad and Unseemly Lady (based on the novel spoilers, it got dark and serious)

Thanks for the recommendations guys! Unfortunately i've already read Negima, Unseemly Lady, and I Married the Male Lead's Dad. I also already watched One Puch Man and Mob Psycho 100 anime too, maybe i'll try reading the manga as well. I'll try reading the others too of course