"Quintet Walls" (クインテットウォール Kuintettouōru) are cards with a special guard ability. They are introduced in Booster Set 1: Brilliant Strike.
When a Quintet Wall unit is placed on (GC) from the hand, by paying [Counter Blast (1)] as cost, the top five cards of the player's deck are revealed, all cards of the same clan as the Quintet Wall are superior called to (GC), and the rest of cards are sent to the drop zone.
You may only include up to four Quintet Wall units in a deck due to the "Sentinel" ability. As Perfect Guard cards also have "Sentinel", you can only use both types of unit in a deck if the total number is four or less (for example, you may use 2 Perfect Guards and 2 Quintet Walls, or 3 Perfect Guards and 1 Quintet Wall).