The Ginko are silver, glowing, blind fish that swim within the dark mushi called Tokoyami.
Prolonged exposure to the light the Ginko gives off will eventually cause a change in the creatures around it. One of their eyes will be lost to darkness (very literally), and the other will turn an unearthly green, while hair (or scales in the case of fish) will turn white. Eventually, the affected will become Tokoyami themselves, with the transformation starting in the one remaining eye and expanding from there. There is no known way to prevent this transformation, not even being kept away from the Ginko's light. While it can be delayed somewhat by unspoken means, this final change is inevitable once the hair and eyes change.
Before the start of the plot of the anime series, Yoki, an 11 year-old boy, was swallowed by the Tokoyami; although he managed to escape, but only after being exposed to the Ginko thereby turning his hair white and one of his eyes green (the other eye he had to give to the Ginko). After the Tokoyami was through with him, Yoki woke up the next morning being unable to remember anything but a name - Ginko. He started to go by that name and became the protagonist of the series.
This mushi, when living with the Tokoyami, is weak to light.