|Spoiler Alert: This page contains spoilers. Read at your own risk.|
He's the very first male student who approached Shiba Tatsuya on the day after enrollment, and has since then been his companion and friend.
Appearance and Personality
Leo possesses Germanic features, chestnut hair, green eyes, and an athletic body. He is also considered good looking and has gathered the interests of a few girls. As time passes in high school, Leo becomes more muscular and sturdy.
He has an easy-going personality and a bright disposition, along with a frank and direct attitude, which often gets him into heated arguments with Chiba Erika. He's also someone who can be easily persuaded, and motivated to do favors. Sometimes, he can be clueless and ignorant in a certain way. He is quite a courageous and a hot-blooded person.
However, as a descendant of a Fortress Series Magician, Leo also carries a deep fear of going insane and dying early, as many of the first generation test subjects did. In an effort to avoid this, Leo tries to follow his impulses, such as roaming at night like his grandfather, who lived a natural life span.
Leo loves his grandfather to the level of continuously using his grandfather's old generation gauntlet CAD since he became a magician.
On enrollment day, Leo introduced himself and mentioned a bit of his background. His father is half-Japanese, and his mother is quarter-Japanese. His grandfather was part of Germany's First Type-Burg Folge (Fortress Series), magicians whose physical abilities were augmented through genetic remodeling, and he was one of the few survivors from the first generation.
Leo is the third generation of "First type"-Burg Folge series. According to Ernst Rosen, apart from his grandfather (Georg Ostburg), every other "First type" died in their first generation itself. Therefore, Leo who doesn't show any signs of mental deterioration and being the "First Type" series is considered a very valuable "subject".
Leo's home is located in Sōka, Saitama Prefecture running private welfare facilities and work placement businesses. By his great-grandfather's time, they followed the old-school type yakuza organization, but this was dropped as Georg Ostburg's condition to marry in as the Saijou family's son-in-law. Being Georg's only descendant who inherited the First Type-Burg Folge genes, Georg personally raised Leo with love and advice to live with the troubles the genes would influence in daily life.
During the 2095 Nine Schools Competition, Leo is seen working at the kitchen of the hotel where the First High School competitors are staying. He later tests the Mini-Communicator that Tatsuya developed and would eventually be recruited by Tatsuya to be part of his substitute team on the First Year's Monolith Code Competition after the previous competitors had to forfeit where his position is to defend their Monolith from the opposing team using the Mini-Communicator and his Fortification Magic. 
Leo possesses a wild nature that endows him with a swift reaction time and a nose for victory. Furthermore, he possesses inhuman endurance, demonstrated during the final match against Third High School in the Monolith Code event, and in his fight against one of the "Vampires" as a result of his ancestry.
A branch of Convergence Magic. Fortifiying Magic stabilizes the relative coordinates of molecules within a narrow area. Leo's Fortifying Magic simultaneously executes the Activation Sequence as well as the design and invocation stages of the Magic Sequence, which spreads out in succession while maintaining a constant state of magic renewal. This process allows his personal CAD and even his own clothes to be hardened to the point where they become similar to a suit of armor in terms of defensive power. Combined with his fists becoming enhanced with Speed and Move Type Magic, Leo can be a truly terrifying force of sheer destruction against his enemies. It is noted that Leo is one of the best (unparalleled) when it comes to Data Fortification. It is also the reason why Tatsuya selected Leo as best suited to use his toy - Mini-Communicator (Weaponized Integrated CAD).
➨ Panzer (パンツァー)
Panzer (German for either tank or armor) is a technique that belongs to the convergence magic system. Regardless of how strong an impact is, the magic fixes the coordinates of the target in order to prevent it from shifting, essentially preventing the target from being broken.
➨ Halt (ハルト)
Halt (German for halt) is a convergence magic technique that fixes the relative object to a certain point, preventing movement by external forces.
➨ Schild (パンツァー)
Schild (German for shield) is a convergence magic technique that applies the effects similar to Panzer to the caster's CAD only to turn it into an enhanced shield.
➨ Streitkolben (シュトライトコルブン )
Streitkolben (German for mace) is a convergence magic technique that takes body expelled psions and converges them upon a body part as a perfect tight cover, creating an iron ball level enforced limb to attack with. It was initially used in the Mahouka SS volume.
➨ Siegfried (ジークフリート)
Siegfried is a technique that hardens ones own body and is Leo's trump card. Siegfried works directly on the body itself, it doesn't apply to objects and can't stabilize relative coordinates like Panzer. It creates a force field that cancels any external force that is inflicted on the body by not accepting external changes in the relative positions of the molecules that make up the body. Unlike Panzer, the magic constantly acts to fix the relative position at this moment when the body decides. It protects against piercing, cutting, striking, pressure, and the majority of chemical and electromagnetic attacks that can't permeate through skin and muscle tissue. Because this convergence magic applies only to the body, the caster's clothes are not protected and resulting possible nakedness or an immoral appearance. During this magic's usage, the body natural seeks to maintain internal body temperature, therefore, massive amount of physical strength is consumed, resulting in hunger. It was initially used in the Side Story Novel.
➨ Usuba Kagerō (薄羽蜻蛉)
The technique relies on Leo's specialty of Fortifying Magic to solidify the flatness of the very thin, transparent, black membrane that is released from the Chiba Family's secret sword "Usuba Kagerō", a weapon made up of a protruding head at the front end that is much wider than the handle and is approximately 10 cm long (from the side, it looked almost like a 50 cm length Latin cross), when activated.  This creates an ultra thin blade made up of carbon nanotubes that are woven into an ultra thin plate that is 5 nano-meters thick, making it sharper than any sword or razor, and can easily cut any armor in half. Usuba Kagerō is essentially Leo's custom made, powerful finishing strike, resulting from highly intense training in a short period of time with Erika.
➨ Old Generation Gauntlet Voice Recognition CAD
Leo initially used old generation voice recognition CAD during his first year at First High School. It was handed down from his Grandfather. It was later replaced with Leo's modern CAD, the Knuckle Duster Thought Operated CAD.
➨ Weaponized-Integrated CAD
Leo used the Mini-Communicator (Weaponized Integrated CAD), during Monolith Code. It was a prototype simulated sword CAD designed by Tatsuya that focused on extending the blade portion of the CAD outward to increase its range. It was described by Leo as "a really long sword". He could extend it outwards or fling it like a projectile. Leo temporarily used this CAD as Tatsuya's lab rat because of his specialty with Data Fortification / Fortifying Magic. Mini-Communicator is also the name of this CAD's only infused magic technique.
➨ Knuckle duster type Thought Operated CAD
- The kanji for Leo's family name means the following: 西 means "west" and 城 means "castle."
- Volume 1, Chapter 2
- Volume 6, Chapter 6
- Erika and Saijou story, Temptation from Rose
- 『満開!ブルーム放送委員会』 第3回（2014年5月7日放送。アニメ『魔法科高校の劣等生』Archived 2014年5月30日, at the Wayback Machine.2014年5月7日閲覧、その後で記載内容に変更あり）
- Volume 4, Chapter 9
- Volume 4, Chapter 10
- Volume 9, Chapter 5
- Volume 2, Chapter 10
- Volume 4, Chapter 9
- Volume 7, Chapter 11