* ~ Japanese high schools run from grades 10-12.  So there are only freshmen, sophmores, and seniors.

** ~ In the manga, Yami no Yuugi does not get any taller.   His outfits don't change either, he just wears whatever Yugi's wearing.

Very little is known about Yami no Yuugi.  The spirit of the Sennen Puzzle has a fierce sense of justice and has no qualms about dishing out punishments to those who need them.  He shares the same love of games, albeit a bit more extreme, with his lighter half, Yuugi Mutou.  At first Yuugi believed he was created by the puzzle, but later learns that the spirit was actually sealed inside it.

Who or what the spirit really is or was and why or how he came to be sealed away is not yet known.
Is he really just the dark side of Yuugi, or is he something more?