Guardians are mysterious creatures that appear as companions to members of the Eto royal family. They can take the shape of a variety of animals, and all have a single unifying feature: their bodies appear to be made of a substance resembling the night sky, full of stars.
Thus far, three Guardian-like entities have been seen: Dusty, a cat-shaped Guardian belonging to Kat; Xii, a crow-shaped Guardian belonging to Raven; and Wolp, a majestic winged deer-shaped Guardian belonging to King Cai. Dusty and Xii grant their owners the ability to control gravity, as does Wolp, as seen when Cai frees Elektricitie.
However, Dusty and Xii are actually two halves of the same Guardian, the Sphinx. Therefore, technically there are only two Guardians, the Sphinx and Wolp, both following a mythological theme.
Due to Dusty becoming weakened upon consuming Nevi-killer poison, it can be presumed that Guardians are a type of Nevi.
During Kat's fall from Eto a hundred years ago, Dusty appears to generate from within Kat, implying that the Guardians of Eto's royal family are born from the individuals themselves. With Guardians potentially being a type of Nevi, this could imply that Nevi are also born from people. This can be taken into further consideration since the "Light Nevi" in Eto are benevolent and will serve under the command of the royal family.
The stars in their body don't move as the player moves around and instead stay in one place, as though looking through a window. This suggests that they don't actually contain any stars and their bodies are just a void or window to another dimension.