"Gin" means 'silver' and "Yami" means 'darkness'.
He was named based on the concept of the specific mushi (spirits of nature) known as
the Ginko ('little silver one') and the Tokoyami ('total darkness')
which are spirits that live in a symbiotic relationship.
Mushi are nature spirits that not everyone can see.
If exposed too often to the light of the Ginko, one may have their hair turn silver-white, lose an eye, and have their remaining eye turn green.
If enveloped by the dark of the Tokoyami, one will become consumed by it and lose their memories.
The Ginko and Tokoyami live symbiotically in that the Ginko lives within the Tokoyami and helps it break down its prey. The light that the Ginko gives off is the resulting product of the Tokoyamis' 'digestion' after having turned the rest of its food into a part of itself.
It is said that to those who can see it, the Tokoyami appears as an eternal sea of pitch-black darkness as it surrounds its victims and that the Ginko within it appears as a great, fish-like spirit of light swimming through said sea of darkness.
In order to escape the Tokoyami, one must surrender an eye to the Ginko and remember a name - any name - to claim as their own. Upon surviving such an encounter, an individual will be left with silver-white hair, a remaining blue-green eye, and no memory but that of the one name they clung to. Survivors also seem to have the strange tendency to attract a great deal of mushi of all kinds to them and must constantly smoke a particular kind of herb in order to disperse said spirits from them less the concentrated, gathered group of mushi become dangerous to the residents of a particular vicinity. The eye socket of a survivor from which their taken eye was removed remains empty and dark; it seems this darkness signifies the space housing a certain amount of Tokoyami, though how much is hard to say. It may be this Tokoyami housed inside the survivor that draws other mushi to them.
One such survivor of these mushi is a man known simply as Ginko - a 'mushi-shi' or 'mushi master' who travels as a nomad to learn more about different kinds of mushi and who must continue to move from place to place in order to avoid having mushi collect about him. He remembers nothing of his past life as a boy by the name of Yoki, nor does he recall the first mushi-shi, Nui, who discovered and named the Tokoyami and Ginko, the latter from whom Ginko the man gets his name. Nui was ultimately consumed by the Tokoyami, and young Yoki very nearly shared the same fate, but she told him to close one eye and remember a name which ultimately saved his life.
Nui and Ginko are the only known humans to have contracted this condition of silver-white hair and a green eye from the Tokoyami and Ginko, though from Nui's research before she died, it seems that animals such as the koi fish in her pond and possibly her husband, son, and whole village fell prey to these spirits. Due to the nature of known confrontations with these very dangerous mushi, it is naturally difficult to determine if there are more surviving victims.
Even with those who display the symptoms/signs/identifying features of silver-white hair and a green eye or two, it's tricky to say if they did, indeed, encounter the Tokoyami or were merely exposed to Ginko light. What's more is that it's unknown if the traits are hereditary, though certain beings seem to have exhibited silver-white hair in a combination with blue-green eyes as well as a tie to memories and a certain affinity to darkness.
Such individuals include (but are not limited to) Riku (Kingdom Hearts), Kadaj and other Advent Children including Sephiroth (Final Fantasy), Towa (DNAngel), Riku Kaitani (Eyeshield 21), Hitsuguya Toshiro (Bleach), Dr.Franken Stein (Soul Eater), Kou Yaten (Sailor Moon), Sue (Clover), Hayato Gokudera (Reborn), Youmu Konpaku (Touhou), and more.
