Tokoyami

The Tokoyami (常闇, lit. Eternal Darkness) are mushi that exist in shadows and come out at night. It eats smaller mushi and people's memories. They also contain the Ginko.

Description
These mushi are created when living beings are exposed to the light emitted by the Ginko. When a living being is exposed to the light emitted by the Ginko for a long time, they turn into Tokoyami. This is also how the Tokoyami came into existence.

If one is ever swallowed by the Tokoyami, first one must keep one eye closed and the other open (the open eye will then be taken by the Ginko). One may escape from this mushi by using this method, but will only be able to remember one thing, such as a name, even if it isn't personal.

Plot
Before the start of the plot of the anime series Yoki, a 10 year-old boy, was swallowed by the Tokoyami. He managed to escape, but only after being exposed to the Ginko thereby turning his hair white and his remaining eye green. Yoki was only able to remember one name, "Ginko", which was the name of the mushi within the Tokoyami. He then went by the name Ginko and became the protagonist of the series. (We see all this in episode 12 of the anime, 'One-Eyed Fish', which is set in an earlier time than the rest of series, and, along with  episode 26, 'The Sound of Footsteps on the Grass', is essentially Ginko's origin story.)

Weaknesses
It has an affinity for Light.