Shikigami is a small series used by Sarina in the Yu-Gi-Oh! GX anime. They are identified by having "Silver Spirit -" or "Magical Mirror Spirit "- in their card names. By now, the only Monster Cards in this series are "Silver Spirit - Ukyo", "Silver Spirit - Sakyo", whose names refer respectively to the right and to the left direction also "Magic Mirror Spirit Asogi" and "Magic Mirror Spirit Nayuta" appear to represent the spirits of yin and yang.
"Shikigami" monsters all Summon their 'partner' monster from the Graveyard when Summoned; this is mostly used to place counters on "Full Moon Mirror" to activate "Infinite Fiend Summoning Mirrors" and then Tribute to Summon "Load of Darkness", and also to Summon Token Monsters to the field.
The monsters in this series are related to mirrors. Their ATK and DEF points are the inverse of their 'partner' (i.e. the mirror opposite). They also Summon each other (as soon as one appears its 'reflection' also appears or you can't have one without the other).
This series has a small collection of related cards comprised mostly of Trap Cards. They can only be activated if "Sakyo" and "Ukyo: are on the field. They generally protect your monsters and destroy your opponent's, which also works towards placing counters on "Full Moon Mirror".