The Eraser, also known as Lenny Fiasco was a college classmate of Bruce Wayne, turned criminal who "erases" the evidence of other criminal's crimes for a portion of their profits.
While attending college, Lenny was known for constantly making mistakes on his assignments, which earned him a great deal of ridicule. He was also in love with a girl named Celia Smith and wanted to invite her to a school carnival, but she went instead with Bruce Wayne.
Tired of all the bullying and failing of classes, he dropped out of college and pursued a life of crime as The Eraser. Dressed in a suit that made him look like a number 2 pencil and an eraser shaped mask, he offered to "erase" the evidence of another criminals crime for 20% of the take.
His helmets eraser was treated with a special chemical compound that could "erase" even the smallest traces of forensic evidence. Batman, disguised as an organ grinder, hired the Eraser for a fake robbery.
Eraser did however recognize him as former classmate Bruce Wayne and revealed he became a criminal because Bruce got the love of Lenny's life Celia.
Bruce did not remember Celia however, sending Eraser into a rage. Eraser used sleeping gas to kidnap Bruce and bring him to a replica of the ice carnival from college. Robin arrived and fought Eraser long enough for Bruce to get in his Batman costume discreetly. Eraser was defeated and sent to jail. Since then he has made only small cameo appearances.
Batman the Brave and the Bold
He first appears in the episode "A Bat Divided" when the three Batmen and Firestorm go to a bar where a bunch of Batman villains hang all the villains attack them. The Eraser lunges for one of them but Batman dodged it and threw a pool rack at his helmet. He then gets slammed into a wall. He later makes a cameo in the episode "Joker the Vile and the Villainess".
The Lego Batman Movie
Eraser makes a cameo appearance in the Lego Batman Movie, as one of the criminals that the Joker employs in his scheme to take over the city.