Leonhart Saijo, known as Leo, is a second-year Course 2 student at First High School in the initial timeline. He has Germanic heritage from his grandfather, Georg Ostburg, a survivor of Germany's First Type-Burg Folge magician program. His father is half-Japanese and his mother quarter-Japanese, giving Leo chestnut hair, green eyes, and an athletic physique at 1.8 meters tall and 75 kilograms. His build grows noticeably more muscular and sturdy during high school.
The Saijo family runs welfare-rehab facilities, vigilante groups, and rental-work placement businesses in Sōka, Saitama Prefecture. They historically followed old-school yakuza traditions until Georg mandated abandoning them as a marriage condition. As a third-generation Burg Folge descendant, Leo possesses genetic enhancements granting superhuman endurance and swift reflexes, demonstrated when he survived a motorcycle collision saving a child in 2093. He harbors deep-seated fears of inherited mental deterioration and early death, leading him to embrace impulsive behaviors like nocturnal roaming, emulating his grandfather.
Leo exhibits an easy-going, frank personality with a courageous and hot-blooded disposition. He frequently clashes in heated arguments with classmate Erika Chiba, though their dynamic lacks romantic tension. While sometimes clueless, he is highly motivated to assist friends, immediately befriending Shiba Tatsuya upon meeting during enrollment. His primary interests lean toward food rather than romance, and he reveres his grandfather, using Georg’s old-generation gauntlet CAD throughout his first year.
Academically, Leo participates in the Mountaineering Club, later becoming a trainer for junior students. During the 2095 Nine Schools Competition, he works in the competitors' hotel kitchen and tests Tatsuya’s prototype "Mini-Communicator" CAD. This leads to his recruitment as a substitute in the Monolith Code event, where his Fortification Magic defends First High’s monolith. He graduates high school and enrolls in Katsuwa Rescue University (Rescue U) to train for disaster response. Post-graduation, he assists in training underclassmen like Tookami Marika at martial arts summer camps, teaching advanced techniques such as the medical magic "Badariforging."
His magical expertise centers on Data Fortification, a branch of Convergence Magic that stabilizes molecular coordinates within a narrow area. This allows him to harden objects or his clothing into armor-like defenses and enhance his fists for combat. Key techniques include Panzer, which fixes target coordinates to prevent breakage from impacts; Halt, which immobilizes objects by fixing them to a point; Schild, which converts his CAD into an enhanced shield; Streitkolben, which covers limbs in psionic force for blunt-force attacks; and Siegfried, his trump card that hardens his body to nullify external forces. Siegfried leaves clothing unprotected, risking exposure, and causes extreme hunger due to energy consumption. He briefly masters "Usuba Kagerō," a technique combining Fortification Magic with the Chiba family’s secret sword to create a molecular-thin blade, but discontinues its use due to legal restrictions.
Leo’s equipment evolves from his grandfather’s voice-recognition gauntlet CAD to modern knuckle-duster thought-operated CADs gifted by Rosen Magicraft. He temporarily uses Tatsuya’s "Mini-Communicator," a weaponized integrated CAD functioning as an extendable bludgeon. He appears in the 2096 spring break incident at Minamitate Island, assisting in rescuing the Watatsumi series test subjects and combating USNA magicians.