Ago was a Kryptonian boyfriend of Kara Zor-El, who would later become Supergirl, before the destruction of Krypton. He was attending school with her in Kandor, but unbeknownst to Kara, he was a traitor. He was sympathetic to General Zod, and even Zor-El, and betrayed the Kryptonian Council's rule to assist Zod's army. (Smallville Legends: Kara and the Chronicles of Krypton)