Yeah I'm incredibly mad right now with my CP not going to even Neutral natured at all, so I made a separate save to test some Shark codes just because I wanna know if the nature is the only thing preventing me from getting Delta Attack.

So I used the Nature changing code found in the "All Codes!" thread, but instead of using the values there, I used 0064, a value way above the listed 0034 for Very Good nature, because of our knowledge here where there are levels above Best nature and below Worst nature.

And to my surprise, my CP only went up to GOOD.

So I was like "huh, maybe I shouldn't have messed around with that value so much..."
So then I used 0034, the exact value listed for 'Very Good' nature in the All Codes thread...

I fire up the game and... my CP turned out BAD!

In disbelief, I fired up and re-entered the code just to be sure, and my CP still turned out BAD!

Now in relation to the things we know about Items, the guy who discovered Jelly effects (DP?) said that these Jellies don't directly affect nature, but increase or decrease an unknown value by 33.

Seeing as how a value of 0064 (100) in the code just yielded me a 'Good' and a 0034 (52) yielded me a 'Bad', there's one possible conclusion we can draw here, albeit someone needs to do a bit more testing on this:

Certain Breeds, determined by either specie or randomly determined on birth (not sure) might have a set '0 point' in nature, where some monsters just have insane trouble getting to the other end of the spectrum because of that value.

For example, let's take the infamous Celious who has an Evil technique but cannot seem to get below Neutral. It may be possible that it's '0 point nature' is really on the positive side, and getting a -100 on the so-called 'unknown value', does not place the -100 directly on the Nature of the monster, but rather, is added up to the '0 point nature' of the monster which is then put as the current Nature of the monster.

I will test other values in the 'Nature' modifier code using this odd CP as an experiment. 0064 is 100 so I'm assuming it was the max I could get, and the negative nature values actually use values way above 100 (e.g. 00A4 is listed for Worst nature, which is 164) which may mean that the game is using either 1's or 2's complement to handle the negative numbers for some of the values.

Can someone take a better look on this? This may just be the final missing step we need in determining the Nature 'bug' once and for all. I'm just assuming from this miniscule test case of a possible conclusion, but I'm not really that good with researching in-depth game mechanics, so... I'll just test with other extreme values and update with some results here.

Maybe one can start by using the nature changing code with 00A4 on a Celious?

...I actually meant Centaurs in general, not Celious. I have no idea why I kept typing Celious

Some more results:

007F (127) - Good
0080 (128... or with how the game treats it, -128) - Worst

007F then looks like the maximum amount of Nature points you can have on a monster, and 0080 would be the completely opposite end of the spectrum.

Gonna check the other values listed in the Codes section, specifically, the ones for Neutral, Bad, and Worst.

...this is looking hopeless for me, it looks like my CP will never be able to learn Delta Attack...

Your results match an observation I made while testing drill results. When changing the nature of a Hare it wouldn't go worst-natured no matter how far I pushed the values towards worst. It might be the breed of monster, or it could just be the monster itself, but some monsters just refuse to have their nature changed completely to one end of the spectrum (best or worst).

Also, DP was correct in that jellies change a value by 33, but he was wrong when he said that it wasn't nature being affected. Nature has several (maybe more, but unlikely) lines of code. What he observed was two lines of code that represent nature apart from another line of code.

Some more things: I was a tad bit curious and started feeding Sour Jellies for 4 straight weeks to my CP that now has a 100 'nature' value (which as said above, only gets it to 'Good'). At the end of it all, he's just stuck at Good.

Very cool, both of you, please keep testing.

Right now I really think that we can look at 2 values here:

1.) Monster's '0 point' Nature Value
2.) Nature Modifier Value

Call them whatever you want but I think this is really what's happening right now as I mess around with more of this.

Easiest example would be meddling with both of the extreme natured monsters: Joker, and Centaur.

I made sure that when the Joker was born, he started out at Worst, same goes for Centaur with best. We can somehow then say that their '0 point' nature is somewhere around -75/+75, something like that, I'm just using arbitrary values here assuming that we can agree that Nature can be measured with a range of -100 to +100.

Using the Nature Modifier codes and putting the Very Good value for the Joker and Worst Value for the Centaur, they ended up with Neutral.

I haven't experimented with Neutral-born monsters, but I'm highly sure Neutral-born monsters are where those Nature Modifier values work properly...

Something like:

'0 point' Nature Value + Nature Modifier Value = Final Nature

Maybe something as simple as that. I don't really have other in-depth ways to test this, but I'm going to assume this until we can find out a different, more plausible theory.

Yeah, this seems on target. Very few monsters are at the extremes.