Natto shouldn't be salty by itself, because salt kills the culture that grows on the soybeans.
Salted soybeans are fermented into miso; unsalted ones become natto.
Normally, you'd season the natto with some combination of strong Japanese-style mustard, soy sauce, scallions or Japanese leeks, and maybe grated nagaimo if you want an even more mucilaginous texture.
The flavor of natto is fairly mild; the aroma is certainly stronger than the flavor itself, and is reminiscent of bleu cheese and sweat.
I'd say it's slightly sweeter than a boiled white soybean would be, but it's possible than an objective measure of sugars might disagree with me there.
Soybeans are very mildly bitter on their own.
Tempeh, a similar cultured soybean, tends to be slightly bitter, but I would say it's not a very pronounced trait, if at all present, with natto, as most of the bitterness seems to be removed by the fermentation.
Assuming you started with frozen natto that wasn't freezer burned that you allowed to reach room temperature, or fresh natto that wasn't excessively old, I would simply mix the natto aggressively in a small bowl for a few minutes until the mucilaginous strands form.
Then season as desired.
If you're expecting a surprising flavor, by the way, you may be disappointed in nattō.
Japanese cuisine emphasizes contrasting textures much more than aggressive flavors, which is to some extent why so many dishes are seasoned only with varying proportions of salt, soy sauce, sugar, sake and mirin, and vinegar (su).
But if you're experiencing an unusually bitter natto, that sounds like a problem with the natto that you purchased, rather than the ingredient itself.
cooking.stackexchange.com/users/2909/JasonTrue
I have tried a number of types of natto, and the smell and the texture are usually much more prominent than the taste.
Recently I had to describe it to a person that had never tried it, and I said something like "feels like runny, gooey old socks, with a hint of raw potato - worth trying once ".
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If we talk just about taste, natto itself is a bit bitter, but it is usually accompanied with a bit of mustard and a light soy sauce which makes it salty and sour.
The smell...
Well, if you can imagine the trash bin, when sometimes there's some liquid in the bottom?
I think we can compare it with that.
At the same time, when you get used to it, it is quite a pleasant taste.
Not everybody gets used though.
Even in Japan.
It's a fermented food and fermented food tend to taste and smell strong, but I wouldn't say it's stronger than a strong French cheese.
Definitely better than Taiwanese stinky tofu.
cooking.stackexchange.com/users/25966/Daniel
Agree with everyone.
Just wanted to add, that my family usually mixes in: dashi-flavoured soy sauce, dash of hot mustard, chopped green onions, bonito flakes, julienned-sized nori, and mix thoroughly in its own bowl until stringy - because the stringiness is the healthy fermented bit.
This way it doesn't taste as bitter and the umami flavour contrasts against some nicer flavours.
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I believe there's a difference in flavour and smell for different people because some are 'super-tasters' and others aren't.
To me it smells like coffee, and to my husband it smells like ammonia - he is a super-taster and finds raw tomatoes, and raw cruciferous vegetables bitter.
I find them delightful.
I make natto at home, and yes, the smell is strong, but to me it smells like badly made coffee from robusta beans.
Acrid?
The flavour is the same with some ammonia, but I kill that off by sprinkling vinegar, stirring, and letting sit for two minutes before serving.
It's very pleasant like that, but I like to mix it with kimchi.
I like to serve it with a runny egg on kimchi Bokkumbap, actually.
cooking.stackexchange.com/users/54574/Gloria
Young natto reminds me of raw foie-gras minus the fat.
Old natto tastes like liver and smells like boiled soybeans mixed with old Brie cheese.
Don't know what people throw in their garbage, but its flavor has never reminded me of that.
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I recently tried nattō on top of rice.  All I tasted was bitter.  I didn't get any nuttiness or saltiness.  The natto was also generously garnished with scallions, maybe that was a major bitter contributor.  What is nattō alone supposed to taste like?
What is nattō supposed to taste like?
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Natto is fairly mild by itself, but is sometimes accompanied by a bit of mustard and a light soy sauce which makes it salty and sour. An unusually bitter natto could be old or have some other problem with it. The smell and texture are stronger than the taste. The taste is quite subjective but it is reminiscent of blue cheese. It can be a little bitter.
Natto should have a fairly mild flavor, like blue cheese or soybeans. Natto can be slightly bitter. The smell and texture are stronger than the taste.