Fat Robin (sometimes mistakenly called Robin) is an overweight teenager that runs around with Fatman, thinking he's a hero.
This is him
Fat Robin was originally known as Loser Loser. He was a circus freak with his family. They traveled across the country impressing people with their lameness. Loser was raised like this, and this was all he knew. But then, one day, Calendar Man was celebrating a holiday, and killed Loser's family. He started eating because he was so upset. Fatman arrived weeks later, noticed that this boy was just like him, and let him be a sidekick.
Using his circus outfit, he became Fat Robin, protector of the overweight, just like his hero, Fatman. Fat Robin turned out to be an amazing hero, despite his lameness.
Several years after becoming Fat Robin, Loser decided he was done working with Fatman, as he was smarter. He quit, and became Bitewing.
A new kid named Weirdo Weirdo became Robin in his place.