Kryptonite Man, or K. Russell Abernathy, is a scientist turned supervillian who is infused with Kryptonite radiation, and thus deadly to Superman and family.
Kryptonite Man was kidnapped and taken to Gotham to be tortured and used by Toyman and Mr. Freeze. Stephanie came to rescue him along with Supergirl but they quickly discovered what the Kryptonite Man was being used for- to power a composite Superman/Batman robot that blasted Kara with Kryptonite and grabbed Stephanie!
Superman and Batman rescued Kryptonite Man and disabled the robot.
More about Kryptonite Man here.