System Creature

System Creature is a term for creatures used for their effect while in the battle zone, rather than their combat strength.

Details
Examples of this include cards such as Cocco Lupia, Petrova, Channeler of Suns or Mist Rias, Sonic Guardian. Many of them are key to strategies with abilities such as Cost Reduction, Draw and Increase Power.

As the effect is a constant Static Ability while its in the battle zone, its effectiveness will rise the longer it remains. Therefore keeping them safe from removal and removing your opponents system creatures can be key to victory.

Cards other than creatures such as Noble Enforcer and Hustle Castle have similar properties.

Most of them have text not detailing the zone where the ability will work, but are generally only affecting the battle zone. Up to DM-09 Invincible Blood the text of "while this creature is in the battle zone" was used, but this was later omitted after DM-10 Eternal Arms.
 * Since they tend to have lower power and can be easily removed, it is necessary to prevent your opponents removal cards or use abilities such as saver from Patrial Flame or Breiga, the Wicked Protector can be used to protect them.
 * Evolution creatures with these sort of abilities tend to be avoided due to the loss of advantage when they are removed from the battle zone. Some Angel Command evolution creatures have strong abilities in this manner, but can have high mana costs.