Kami traditionally have two aspects, called the aramitama and the nigimitama. “Mitama” means spirit or soul, while “ara” means wild and violent, and “nigi” means calm and peaceful. “Aramitama” could be translated as “wild spirit”, and “nigimitama” as “calm spirit”.
As kami are often thought of as spirits, it might look as though the aramitama and nigimitama are almost separate kami. Indeed, they are sometimes treated that way. At the Naikū of Jingū in Isë, for example, there are separate jinja for the nigimitama and aramitama of Amaterasu Ōmikami. The main jinja enshrines her nigimitama, while Aramatsuri no Miya enshrines her aramitama. The Gekū is similar; the main jinja enshrines Toyoukë Ōmikami’s nigimitama, while Taka no Miya enshrines her aramitama. The jinja enshrining the aramitama are a few minutes’ walk from the main jinja in both cases, and Kashima Jingū, in Ibaraki Prefecture, has a similar arrangement. At Kashima, the jinja enshrining the aramitama is a few minutes’ walk into the woods behind the main jinja. The jinja can be even further apart: the nigimitama of the Sumiyoshi kami are enshrined in Sumiyoshi Taisha, in Osaka, while their aramitama are enshrined hundreds of kilometres away in Sumiyoshi Jinja in Shimonoseki, at the western tip of Honshu, the main island of Japan.
Jinja that enshrine the nigimitama and aramitama separately are, however, the exception, rather than the rule. I suspect that a large part of the reason is practicality: to enshrine them separately, you must have two jinja buildings, and space to build them, which doubles the cost of a jinja.
The nigimitama is often described as the peaceful aspect of the kami, bringing blessings to people, while the aramitama is violent and active. Some matsuri directed at the aramitama have the goal of calming it down, and returning the kami to her nigimitama. However, sometimes violent action is necessary, and in those cases a matsuri would be directed to the aramitama. Because people do not, in general, want their lives to be violently disrupted, the nigimitama is generally more popular, and regarded as the main aspect of the kami. The aramitama is not, however, any sort of evil spirit.
My interpretation of the two aspects is as follows. While this is largely personal, I have read similar interpretations from other people, so it is not far outside the mainstream. A kami’s nigimitama favours the status quo. The nigimitama is a force for stability, but not stasis. The status quo is not unchanging, after all: people mature, and learn new things, and new houses are built. Much of that is part of the status quo However, that growth and development happens within the boundaries that are already set.
The aramitama, on the other hand, favours change. The change doesn’t have to be instant, but it goes beyond what was expected and predicted. This change disrupts the established order, makes plans impossible to carry out, and creates new options.
Most people want to avoid too much unexpected change. It is hard and stressful to deal with, even if it is ultimately good. The rulers of a country like unexpected change even less, as it almost always reduces their power. Further, many of the obvious examples of unexpected change are negative: natural disasters, plagues, deaths. This explains why the nigimitama is, and always has been, more popular. On the other hand, it is obvious that, sometimes, the aramitama’s intervention is what you want or need.
One of the reasons I like this interpretation is that it is clear that neither the nigimitama nor the aramitama can be described as simply good or evil, while still providing strong reasons why the nigimitama should normally be favoured, in line with what we know of historical practice. It also seems to fit with what information I have about how the two spirits were treated.
However, that raises one of the issues here. Although this distinction seems to have been very important in the past, it is much less important now. Most introductions to Shinto mention it in passing, if at all, and even at jinja that have separate sanctuaries for the nigimitama and aramitama, the difference is not emphasised. This distinction is not a very important element in contemporary Shinto practice. Indeed, I am not even sure that all kami are regarded as having these two spirits; I do not think I have come across any references to O-Inari-san’s aramitama, for example, and although Tenjin-sama went from a dangerous kami of lightning to a benevolent kami of exam success, I have not seen that cast in terms of the aramitama and nigimitama. It may be safest to regard this distinction as part of the history of Shinto, rather than of its contemporary form.