The Kryptonite Kid
In September 1960, the telepathic Kid and his Kryptonite Dog pass through a green cloud in space, both gaining an emerald hue and the attributes of Kryptonite, deadly to Superboy and all survivors of Krypton. The duo journey to Earth and use their Midas-like ability to transform everyday objects into Kryptonite, threatening the lives of Superboy and Krypto--who are only saved at the last minute by the 5th-Dimensional magic of Master Mxyzptlk (SB No. 83/1: "The Dreams of Doom").
The Kryptonite Kid returns sometime later, having been released from a 5th-Dimensional prison, and using his Kryptonite touch, he paralyzes Superboy. The Kryptonite Kid gives the Boy of Steel an opportunity to escape by exchanging Kryptoâ€™s life for his own. Superboy finds Krypto and turns him over to the Kryptonite Kid. However, after a short chase, Krypto appears unaffected by the Kryptonite. He then pushes the Kidâ€™s ship into a cloud of Red Kryptonite. The Red Kryptonite changes his skin color to a reddish hue and causes him to act benevolent and friendly. Superboy explains that Krypto was just a robot, and the events were planned. The Kid leaves Earth, but Superboy realizes that the effects of the Red Kryptonite are only temporary, and his enemy will one day return (SB No. 99/3: "The Kryptonite Kid").
Later, the villain returns to threaten the Man of Tomorrow as an adult, now calling himself the Kryptonite Man (S No. 299, May 1976: "The Double-or-Nothing Life of Superman!").
In a subsequent adventure, a second and different figure, also calling himself the Kryptonite Man and a refugee from Krypton itself, seeks to destroy the Man of Steel and Supergirl (S No. 397, Jul 1984: "The Born-Again Kryptonite Man!", SG2 No. 21, Jul 1984: "Curse of the Kryptonite-Man!").