The Ginko are silver, glowing, blind fish that swim within the darkness of the Tokoyami.
Prolonged exposure to the silver 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 and was 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). Ginko was victim to both the Tokoyama and the Ginko, and after being spared by these mushi, having sacrificed his eye and memories, the only thing he was able to remember when he woke up the next morning was the name "Ginko", which he adopted as his given name.
This mushi, when living with the Tokoyami, is weak to light.