The Unique Rule (ユ二ークのルール, Yunīku no Rūru) is a rule in Genso Suikoden Card Stories that restricts the cards you can play.
In its most basic form, the Unique Rule states that only a single Character card of a name can be played at any one time by either player. Each player cannot play a card of the same name, nor can a single player play two cards of the same name. This does not restrict the deck, which can contain 4 cards of the same name, merely what can be played on the field.
The rule does encompass different numbered cards with the same name. So a later edition card cannot be played alongside an earlier release of the same card, even if their attributes and abilities differ. It also applies to cards with multiple names in them. For example, you cannot have a McDohl & Germio card and a McDohl card, or a Gremio card, on the field simultaneously.
The rule does apply to Mastermind cards. You may not have a Mastermind card and a regular Character card of the same name in play. If your opponent plays a Mastermind card, you cannot play a Character card of the same name either. The Unique Rule does not apply to Mission or Facilities cards.
The Betray conditional takes advantage of the Unique Rule by replacing a Character card with the Betray conditional your opponent has on the field with the version you played, forcing the original card to be discarded.