Shadow Soldiers

The only things that have been confirmed is that being eaten by one prevents a person from being reset, and that being touched by one saps one of their energy, and burns like fire. It is possible that each one represents a person who has died and not been reset, as is seen with Kodya in chapters 53 and 54.They appear with greater frequency as the Shadow Hour approaches. Once the Shadow Hour comes, a large number appears.

Be aware that there are spoilers in this article.

Abilities
Their appearances are marked by their loud screeches and the cold that comes with them. However, Gyrus is immune to their cold.

They can stretch out their bodies so that they can manipulate their surroundings and attack those they perceive as an enemy. They can also complete a boss sword as seen by Shadow Kodya in chapter 54.

Being touched by one drains a person of energy, and leaves a feeling of a burn.

Their movements are quite janky and unpredictable, and they seem to flicker, except it's a shadow, not a candle light.

Their shapes resemble the people they once were before they were consumed. This was proven when Kodya became a shadow and the shadow resembled him.

If a living being is completely engulfed by One, they will become a Shadow.

Only Boss Swords and Gyrus can harm them. They are immune to all other attacks. Nothing can kill them.

They appear to be a hivemind. The shadows seem to have no recollection of their past selves, they are stripped bare of their memories and absorbed.

They speak a different language made of symbols that only Gyrus seems to be able to comprehend.

They seem to be attracted to boss swords, which, ironically, is the only thing that can harm them.

Not much is known about the shadows, but we can infer information.