Superman, real name Clark Kent, is Supergirl's cousin. He is a Kryptonian and was sent to Earth to escape the destruction of Krypton like Supergirl, and was raised up by the Kents and graduated Super Hero High long before the arrival of his cousin.
Superman is a big strong man with a spiked up quaff and huge chest muscles. He wears a t-shirt with a House of El crest on it and cape.
He appears as a statue behind some graduates.
He is mentioned again by Supergirl when she's doubting her abilities.