Djedefre became pharaoh after his father Khufu died. Unlike his dad, his brother Khafre, and his nephew Menkaure, Djedefre didn't build a pyramid at Giza; he chose to build at a site called Abu Roash instead. No wonder you probably never heard of him!
Djedefre was the first pharaoh to add the title "Son of Re" to his birth name.
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Red granite head of Djedefre. By Louvre_032007_19.jpg: Neithsabes derivative work: JMCC1 (Louvre_032007_19.jpg) [Public domain], via Wikimedia Commons
Ruins of Djedefre's Pyramid. By No machine-readable author provided. AhlyMan assumed (based on copyright claims). [Public domain], via Wikimedia Commons
Watch: "The Lost Pyramid of Djedefre"