Billy, the Seal Boy was the stage name of a former circus performer.
Billy had flippers for arms and legs, and performed in a show much like a seal with a ball. He quit the circus life and lived with several other former performers on a quiet farm.
The Seal Boy is the only character from Detective Comics #410 to make more than one appearance. In recent issues of Shadowpact, he has become the proprietor of the Oblivion Bar.