Soul Corruption - Connection and Rank
While a Soul-Corrupted person does get "infected" by Yo-kai, not all of them will result in a transformation or a personality change. In fact, whenever a Yo-kai can do so (consciously or accidentally) depends on how Connected the person is to it.
Each person's soul is different, and that involves corruption too. While a few specific Yo-kai's essences might corrupt someone easy, the same essences might not work so well with another person, to the point that some don't even manage to enter in the first place.
For example, Jibanyan's essence connects well with Nate, it isn't as effective with Katie, and viceversa in the case of Komasan's essence. Snottle's essence doesn't even work at all for either of them, although another afflicted might connect with it very well.
As a general rule of thumb, think of it as a few essences that connect very well, a great amount that simply works and a bunch that works badly (or not at all).
Another thing to pay attention to is the Rank of a Yo-kai. The higher the rank, the stronger and more forceful the influence is and the faster it takes to build up.
E and D rank are subtle and take quite a bit of time to build up enough to take control. In fact, E-rank essences are also easy to resist. They do feel annoying, though.
C and B ranks take less time and are more forceful, taking a day or two to fully build up. The corrupted might need a more conscious effort to resist the influence. They tend to be bothersome, a more pronounced feeling.
A ranks are fast, usually taking just a few hours to assert themselves. With those, it almost seems like the afflicted is actively fighting a possession from a spirit, which is not a wrong comparison. This is where it starts to get quite mentally straining.
S ranks... more than a corruption, it really resembles a possession for the unexperienced. Very forceful, they tend to take over in a few minutes. It almost looks like a horror movie scene, head-holding in a fight for control, heavy breathing for the changing body and all that jazz.
While these are a rule of thumb, it also depends on the person, on the amount of essence and how experienced one is with their condition. Willpower also helps a bit.
