I'm looking for manga with character(s) that are Crazy, but appear stable most of the time, or at least keep it hidden to most people. Doesn't matter the genre

Dame Daze... Zen Zen Dame Daze

Maybe Tonari no Kaibutsu-kun could be fit in your description
The genre it's shojo romance

Sounds interesting, much appreciated for the rec

Dame Daze... Zen Zen Dame Daze

Craziness. Sanity, or lack-there-of is is something I have a morbid fascination with.

First, there's the kind of crazy no one can argue, the psychopath:
Mars might fit. The more I think about it, it does, particularly towards the end. Somewhat melodramatic, somewhat mature shoujo romance, but a classic. The characters are pretty good, and the art is nice for its time, as a plus.

Kubera. It's a webtoon, but still pretty good. This one fits your requirements almost too well. It might tire your patience, because there will be approx. 150 chapters before you realise something is wrong. And yeah, no spoilers, but it's a shock - all the more so because it fits too well - too well from the start.

Good ones in this are hard to find, but I love the idea of them. More often than not, it's the antagonist with the screw loose, but those are typically ones that fade from memory.

Next, there's the normal characters that just send a chill down your spine. They are not crazy, but the panel where you realise you don't want to cross them - because they very well won't hesitate - are awesome. Even if you don't believe they're what you're after, they're awesome:

Bokura no Kiseki People do believe the MC is making things up for the first 15 or so years of his (second) life. And then, like he has believed all along, there comes proof that he was indeed re-incarnated, and with it, it brings nonsensical anger and grief and regret and the need to blame and yada yada, all this psychological stuff.

And Chang Ge Xing. The MC is mad with with grief and the desire for revenge, it mellows with time, but wow there are panels one cannot forget. It gave me chills, but arguably doesn't fit on this list.

For more webtoons (which I have been into recently) there's also two more that may fit. I haven't read enough of them to rec for quality, but I'll still list them:

Cheese in the Trap A romcom where the male MC displays yanderish/stalkerish tendencies. He definitely has a screw loose, but everyone aside from the female MC are fooled into believing he's, well, perfect.

And Bastard (HWANG Youngchan). I've only read the first chapter, but it's clear enough the theme: the psych of a mass murderer.

I tried, but honestly, this isn't my forte. I scratched the barrel, and tried to stay away from those multitudes of supernatural ones where everyone thinks the protagonist is loopy but it's actually real.

Thanks for the recommendations! I'll definitely read these with the amount of extra detail you put into describing them :^)

Dame Daze... Zen Zen Dame Daze

Aku no Hana - the character is regarded as strange by her classmates though. Something like schizophrenia is implied.

Flowers of Evil and Watashi no... Megane-kun - creepy obsessive love genre.

Freesia maybe, the character may be too out to lunch. He's functional though.

Ichi the Killer - Again maybe a bit too crazy and maybe too isolated from the other characters to meet your criteria.

I think Hakuji is another obsessive love one but I don't remember the details.

If you don't mind GL, then I thought the main story in Kuchibiru ni Suketa Orange had a very creepy protagonist. Funnily enough, the story paints her actions in a romantic light and her love interest seems to find her adorable, so I'm not sure whether you'll like it.

Perhaps Happy Sugar Life. I haven't read far enough to be certain whether she manages to keep her insanity under wraps, though.

Mahou Shoujo Ikusei Keikaku (Novel) has characters that appear imminently sane... Until they think their lives are in danger, whereupon they go completely nuts. The novels are ongoing, so you may want to take a look at the anime adaptation first.

The Higurashi no Naku Koro ni anime franchise has members of the main cast going off the deep end every now and again as one of its major plot points, but note that because of the nature of the plot the one who drops the sanity ball switches around every few episodes.