As an item
As the name would suggest, the item protects the fighter's rear from harm. The shield lasts for 30 seconds, with time reduced based on how much damage it has taken (5 Home-Run Bat swings at 30% damage reduces it by about 5 seconds, for example). Some fighters can also take advantage of their back aerials as they won't be damaged from colliding with another fighter's attack. There is a slight push effect near the shield.
In Smash Tour
The Back Shield appears as a red item: when selected, it makes the user start a battle with the Back Shield equipped.
In-game description from Super Smash Bros. for 3DS
This shield protects your back from various attacks. However, it'll break if it takes too much damage.
This durable, defensive shield comes to Smash Bros. from Kid Icarus: Uprising. If you pick it up, it will attach itself to you, protecting your back from most threats. It won't stop grabs or piercing attacks, however, and it only lasts so long before vanishing.
A floating shield from Kid Icarus: Uprising. Pick it up and it'll protect you from attacks from behind. It's not perfect, though, as it can't block grabs or piercing attacks. Still, it'll block any standard attacks until it eventually disappears, so it's a pretty useful thing to get your hands on!
As an item
The Back Shield returns in Ultimate. It retains its design and function from SSB4.
While the Back Shield itself does not have a Spirit, the item appears as an ability for the Primary Spirit Alm & Celica.
Equipping the General Pepper, Alolan Exeggutor, Gyromite, Phyllis & Pelly or Flow Support Spirits will equip the player with a Back Shield at the start of a match.