Kei Sakurai, known by her demon name Leonhard August or The Lion's Heart and Sword, hails from the Sakurai bloodline historically tied to the Obsidian Round Table in World War II. Orphaned young, she was raised by her older brother Kai Sakurai and Beatrice Waltrud von Kircheisen, forming deep familial bonds with both. After Kai and Beatrice died, Valeria Trifa recruited her into the Longinus Dreizehn Orden to fill Seat V, previously Beatrice's position. Valeria promised that Reinhard Heydrich's ascension to Atziluth could revive them, leading Kei to undergo a decade of rigorous training under Valeria for the Day of Wrath. Her combat power stems from the Die Ewigkeit formula. At the Yetzirah stage, she wields Scharlachrot, a flame-enshrouded sword capable of instantly evaporating water and melting steel. Upon reaching Briah, her manifestation "Man sollte nach den Gesetzen der Götter leben" transforms her into living flame. Battle parameters consistently rate her Attack, Defense, Magic, Agility, and Equipment Synchronization at level 2 in Yetzirah and level 3 in Briah. In specific narratives, particularly the Marie Route climax, she temporarily wields the Wewelsburg Longinus—a relic connected to her lineage—during a battle against Eleonore Von Wittenburg. This grants overwhelming power, but the severe injuries she sustains result in her death shortly after overpowering her opponent. Her personality reflects profound loyalty to Kai and Beatrice, coupled with a volatile temper and indomitable fighting spirit, fueling her actions across story routes. While absent in spin-offs like Kajiri Kamui Kagura, her legacy persists through Tenma Morei—a combined reincarnation of Kei and Beatrice—who emerges as a member of the antagonistic Yatsukahagi faction. Her name etymologically signifies "firefly" (螢), reflecting themes of transience and luminosity within her narrative.