BNK3R is a main story boss in Borderlands 2. He was created by Handsome Jack to stop the Vault Hunters from reaching Angel.
The Vault Hunters managed to track Angels location to a Bunker, located inside Thousand Cuts. The Vault Hunters are attacked by Hyperion on the way, and fight their way up to The Bunker. On arrival at The Bunker, The Vault Hunters must defeat 11 Laser Cannons that are guarding it. It is then revealed by Handsome Jack that The Bunker is not a place, but a robot named BNK3R created by Handsome Jack to kill The Vault Hunters.
 Defeating BNK3R
BNK3R has a few main weak spots. Anywhere on his body that is red (i.e. His eye, His back etc.) is most likely a weak point. His turrets are also a weak point, but it is advised not to shoot them, as it causes a large, indestructible turret to appear on his back, that deals heavy damage.
BNK3R is easy to defeat using Zer0's Bore Skill, by shooting at him with a gun that shoots multiple bullets at once, hoping for one to bore through him to deal tons of damage.
BNK3R drops a ton of loot. When defeated, loot will spray all across his arena, and he will continue to drop Green and White Guns for about a minute after defeated.
 Unique Loot
BNK3R has a small chance to drop either The Bitch or The Sham, which are orange items.