Ghost

Soul Manipulation: Soul is the life force that exists in most living organisms in Hallownest. The Knight can absorb Soul from fallen enemies or the environment to cast spells and heal their shell, indicating a unique interaction with this universal force.

Shade Form: Upon death, the Knight's shell shatters, and their raw Void essence, known as a Shade, appears. The Shade is tethered to the location of death and the player must defeat it to recover their collected Geo and rejoin their shell and Void, a mechanic similar to retrieving a body in other games.

Lack of Will/Mind: The original purpose of the Vessels was to be "pure" beings, completely empty and devoid of will, mind, or voice, making them an ideal, uninfectable vessel to contain the Radiance. While the Pale King's plan ultimately failed (as the Hollow Knight had an "idea instilled" in it, likely love), the player character Knight is drawn back to Hallownest, suggesting a unique form of determination and latent potential that allows them to overcome this design flaw.

Void: The core substance of the Knight's being is the Void, a black liquidy substance which can be given form. All Vessels were born in the Abyss from the union of the Pale King (a Wyrm) and the White Lady (a Higher Being related to plants/spores). The Void within them makes them resistant to the mental manipulation and psychic "Infection" caused by the Radiance.

Shell: The Void is contained within a physical shell, which gives the Knight its distinct appearance (small, black body, and a white, horned mask-like head). This shell can be damaged, but is repaired by using Soul.

No Organs: The Knight is explicitly stated to lack internal organs. They do not seem to age in a conventional sense; the main difference in size between the Knight and their older sibling, the Hollow Knight, is due to their environment and training, not natural growth.