Kru-El was a member of the House of El and cousin of Jor-El. He has appeared in different eras in the Superman comics as the most vilest member of the House of El and clashing against his family members. Mostly, he has been know as one of the many inhabitants from the Phantom Zone.
In his younger times, Kru-El was a thorn in the life of young Jor-El, but meanwhile Jor-El became an important scientist, Kru followed a criminal path. He made weapons on Krypton which were forbidden and was put in the Phantom Zone, ironically survivng the destruction of his world. During this era he clashed later no only against Superman but also against Supergirl.
Along with Faora, Kru-El was one of Hounds of Zod, an underling of General Zod in a pre-crisis like Krypton which had been recreated in the Phantom Zone. He was set on the trail of Superman and Lois Lane, both trapped on that version of Krypton. However, this Kru-El still had a heroic trait, showed when the fanatics of the Order of Rao took over Krypton and Kru-El joined hi s family to fight against them.
Previous to the destruction of Krypton, Kru-El tried to kill his cousins, Jor-El and Zor-El along with the daughter of Zor, Kara, to reclaim the leadership of the House of El. His plan was foiled by Dru-Zod, and after the trial, he was sent to the Phantom Zone, to Jor-El regret.