Alright, if you've checked the Item List at my site, you might have noticed the bit about the Jellies, and how they seem to change the nature of a monster.
Here's what I got from my research.
I grabbed a Zuum from the Market. Starts out Neutral. I began feeding him Sour jellies, assuming they make a monster Evil. After 2, it went from neutral to Good. Not good, I wanted an *eeevil* Zuum (Is it an eeeevil petting Zuum?). I fed it a Sweet Jelly and it reverted back to Neutral.
Okay, so Sweet Jellies make them evil (I guess they spoil them) and sour ones make them good. I kept feeding it sweet Jellies, but it wouldn't budge from Neutral. The first time it Cheated on a drill, I didn't scold it, and it went straight to bad (other monsters I've done this with needed 2 or even 3 cheats before they'd move). After a few more jellies and another cheat, it went to Worst.
So, there are a few conclusions that can be drawn from this. Keep in mind that this is nowhere near definite, there is still alot of experimenting to do:
Conclusion 1: Jellies do make a monster good/evil, but monsters have a certain limit to how far it will affect them. I assume that more sour jellies would have pushed it to best, but sweet ones only made it Neutral.
Conclusion 2: each monster is affected by Jellies in a different way. Kind of ties in with C1, but could mean that some monsters could run the full gamut with just a few jellies.
Conclusion 3: Jellies don't affect nature directly, but push tendencies to change. Thus, feeding the proper jelly doesn't change the nature itself, but when you feed the jellies and then do things that do change nature, those things have a more pronounced effect.
Conclusion 4: Jellies have nothing to do with Nature, and this was all just coincidence seeing as I've only tested it on one monster.
I find that feeding enough jellies will eventually push the monster where you want it, but jellies in combination with training methods is far faster.My conclusion comes mainly from my experience with a (Kato/Gali) that I was trying to make evil, starting at best. After 9 sweet jellies his nature became good. I stopped scolding him for cheating or failing, and it only took 6 to get him from good to neutral. The pattern was similar from then on- sweet jellies every week, no scolding, giving it what it asked for. I'm going to try the experiment again with a Heel badge in my inventory, and I'll post the results.