The Birthday Boy

Ray Salinger, a.k.a. the Birthday boy was a serial killer who operated in Gotham before and at the start of Batman's crimefighting career.

Background
Birthday Boy is fascinated by young girls resembling his first kill, a 15-year-old debutante named Amanda Grant. When in his "killer mode", he wears a burlap sack with holes cut out for his eyes, and a birthday party hat, usually in a room decorated like a young girl's room at a birthday party. As a calling card, he left birthday candles where he stole away his victims, and before murdering them told the girls to make a wish to a cake with the words "Happy birthday Amanda" on them. Before the story, he was escaped from the Crane Institute.

Events of Batman:Earth One
As Batman begins his hunt for Jacob Weaver, Birthday Boy is being hired by Mayor Oswald Cobblepot to eliminate various enemies, and at the same time as the banquet to celebrate Gotham City's 300th anniversary, he was killing another victim. After the party, Cobblepot, wanting to get rid of Weaver, sent Birthday Boy the oldest daughter of a judge who failed the mayor, as payment to kill Weaver. Weaver was soon murdered at his apartment, before being sent out to kidnap james gordon's daughter barbara.

At his hideout in Arkham Manor, Salinger was about to kill Barbara Gordon, who managed to break free and stabbed him in the leg with a broken chair piece. thoguh as he walked toward her and was about to slice her throat with a butcher knife, Batman bursted through he wall, and the two start fighting. barbara's father managed to shoot him in the shoulder before being thrown down, Birthday boy was grabbed by batman. The two then tumble through another wall, onto weakened floorboards to the living room. Harvey Bullock tried to stop the fight, only for Birthday Boy to send him into the basement where the killer threw all his victims away. Batman however manages to get behind birthday boy and suffocate him to unconsciousness with his cape. Salinger is then handcuffed and later thrown in custody by the Gotham police after cobblepot's death.