From the Super Mario Wiki
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The Back Shield is an item from Kid Icarus: Uprising that also appears in Super Smash Bros. for Nintendo 3DS / Wii U. When it is picked up, it appears behind the fighter and protects them from most attacks from behind; it does not defend, however, from grabs and piercing moves. The more it is attacked, however, the less time the shield remains active. The Back Shield stays behind the player no matter what direction they are facing.