Lightside and Shadowside forms are two different yet related appearances most Yo-kai can assume either willingly or through the use of the Yo-kai Arks and the Yo-kai Watch Elda by its accepted human.
Lightside forms are the standard form used by Yo-kai outside of battle in most cases; this form serves as the new standard form for Yo-kai in a similar manner to present Yo-kai, and as such this form allows for normal communications with humans, for it has a much friendlier physical appearance. Shadowside forms, on the other hand, are mostly used for combat, and as such most Yo-kai in this form assume a much fiercer-looking and monstrous form than the Lightside form, in addition to being more suited by combat. In both forms, the voices, as well as some personality traits of most Yo-kai, are notably different from each other.
In some occasions, such as Jibanyan, some Yo-kai can revert to their original selves upon experiencing a strong influx of memories related to their previous selves, implying that the transition can be reversible, albeit temporarily.
The phenomena of Lightside or Shadowside forms began at some point in the 30 years prior to the Shadowside movie and series; however, the exact circumstances, as well as the nature on how the transition began, are so far unknown.
Notably, Yo-kai from the original series (with some exceptions like Slimamander/Micchy), retain their original names, and simply distinguish each form by adding Lightside or Shadowside depending on it (in PuniPuni, these terms are shortened to Light and Shadow, alluding to the words when a Yo-kai Ark is turned on either side). Yo-kai which first appear in the Shadowside series (with exceptions like Robonyan 00) have two different names to distinguish each form.
In Yo-kai Watch 4, the mechanics are used in the following ways:
When viewing a shadowside Yo-kai in the Medallium, you can press the R joystick to change forms.
When in battle, if a Yo-kai is in its Lightside form, it can change into shadowside via pressing X+a while playing as it with a full Soultimate gauge, after which, it will revert after a while. While in their Shadowside forms, the yo-kai can unleash a soultimate via pressing X=A again.