Kranna

Carol Benson hatches a ruse to be used on Supergirl, to get her to create diamonds for her from lumps of coal. Disguising herself as “Kranna”, appearing before Supergirl in a rocket actually created by her father, a scientist, she claim to be her long-lost older sister from Krypton. 
 
Carol fakes super-powers (demonstrates she is super strong) with scientific tricks and, finally, gets Supergirl to squeeze coal into diamonds in a super-power “competition. Supergirl leaves momentarily, baiting Carol into revealing her true nature when the Girl of Steel is out of sight.