Martha Wayne Jr. is the daughter of Bruce and Janet Wayne. Her twin brother is named Thomas. As toddlers, they were kidnapped by the Joker, who with the help of a hostage scientist, turned them into bats. He needed money and thought taking hostages was safer. He held them for two billion dollars. Naturally, Batman was upset and the entire Justice League searched for them (in secret, of course.) After they were rescued it was discovered their genetics had been severely altered, and they could not stay human for more than an hour.