|Spoiler Alert: This page contains spoilers. Read at your own risk.|
Chiba Erika (千葉 エリカ) is a student of First High School, a third-year Course 2 student. She is the youngest daughter of the Chiba Family, one of the families at the pinnacle of the Hundred Families.
She is Shiba Tatsuya's classmate and not long after became a friend and constant companion of the Shiba siblings.
Appearance and Personality
Erika is described as a beautiful young girl whose short, bright red-orange hair, which became longer over the time is worn in a ponytail, and has distinct facial features that amplify the impression of her being a lively girl.
She is bright, optimistic, and a person who gets carried away easily. Even for people whom she has just met, she often addresses them in a friendly manner — Tatsuya thought of it as being 'overly familiar' — of speaking. Her frank and seemingly arrogant attitude, however, often gets her into countless arguments with her fellow classmates but only a few people can put up with her, Saijou Leonhard in particular.
She has described herself as someone who, during elementary school and middle school, had spent most of her time alone. She has a pleasant attitude and could easily get along with anyone, but she turns negligent towards relationships quickly.
Her old friends call her cold and say that she acts like a whimsical cat, while others mentioned that she acts high and mighty. There have been many boys who wanted to be in a relationship with her.
She has a habit of talking to herself, but has hid it as she entered high school.
Erika is a member of the Chiba family, which is one of the Hundred Families. In her family name, there is the number Thousand (千).
She is the daughter of the head of the Chiba family and Anna Rosen Katori, her father's lover. Erika was born during the time when her father's legal wife was ill. She hadn't been allowed to use the family name of 'Chiba' until after her mother had passed away and did not receive the name of 'Chiba Erika' until she took the high school entrance exam.
It was revealed that she lives alone in a separate building from the Chiba main household and holds contempt towards her father. She has two older half-brothers, Toshikazu and Naotsugu, and an older half-sister.
Yoshida Mikihiko is a childhood friend from before they entered First High School. It seems they are aware of each others situations, maybe due to their same circumstances. There was a short foreshadow about Erika's past.
|“||The last time he saw this expression was before she cut her hair short, when it was even longer than when she allowed it to grow out last spring. Two years ago, she never loosened her grip on the sword in her hand...||”|
—Mikihiko's recollection, Volume 4, Chapter 10
It's known that Defense Officials and Security Officials appreciate her contribution as a daughter of the Chiba Family. Though it seems that the Chiba Family is reluctant to use her as the poster girl, including the Family Head, Jouichirou. It's been half jokingly remarked that she's the "secret weapon" of the Chiba Family, the "Phantom Sword Princess of the Chiba". Details from the Yokohama Incident are known among the Ten Master Clans. 
A German family that heads a prominent CAD development company. They bear resentment towards Japan for Erika's grandfather's elopement with a Japanese woman, and haven't invested any family members in the Japanese Branch of Rosen until now.
Due to training from a young age, Erika has highly honed reflexes. During Scorched Halloween, she was the only one among the girls who was able to keep up with the physical speed of the boys. Her physical speed is said to be the fastest or at least one of the fastest in her family, thus making her the only one in her family able to use certain secret technique.
- Swordsman's Eye (形状記憶棍刀)
|“||Those movements... the same as Lina's... You guys are Stars, right?||”|
—Erika to Canopus
- As a result of her training, she has developed the "Swordsman's Eye", which allows her to accurately assess her opponent's power and predict her opponent's moves. During the Watatsumi Series rescue, Erika battles against Benjamin Canopus where she immediately sees how dangerous it is to cross swords against his weapon and immediately becomes aware of the similarity in fighting styles of Canopus with Angelina Kudou Shields to deduce that he is part of Stars. Canopus confirms this and also mentions that Erika must be the Chiba swordswoman that Lina mentioned in her report who can fight on par with her. 
As a Course 2 student, her skill with Systematic Magic, at least in invocation speed, complexity of spells, and power & scale of Zone Interference, isn't as great as some other Magicians. However, she seems to be highly adept with magic related to combat. She specializes in Speed-Type (possibly Movement-Type) and Convergence-Type Magic.
- Self-Acceleration (自己加速術式)
- A basic Speed-Type Magic (or possibly Movement-Type Magic) that enhances the speed of the user's physical movements.
- Fortification Magic (硬化魔法)
- Also known as Convergent Systematic Reinforcement, this Convergence-Type Magic reinforces the position of one physical object in relation to another. Erika uses this to strengthen her hollow baton-shaped CAD to the point that it can cut through solid metal. In order to avoid feeding a steady stream of Psions into her CAD and wasting her energy, Erika states that she uses this only at the time of extension and time of impact.
- Yamatsunami (山津波)
|“||Necessary components also included the ability to sprint forward without losing one's balance due to loss of inertia and the ability to hold the blade steady and prevent wobbling. Finally, and perhaps most importantly, the high speed reaction and perception that would not be subverted by the loss of inertia. Those were the critical components to Yamatsunami. Erika was born with that "speed" and with countless days of harsh training, she finally obtained this ability.||”|
—Describing the necessary components of Yamatsunami, Volume 7, Chapter 11
- A Speed-Type/Weight-Type Magic that first minimizes the inertia of the user and the weapon while closing in on the target at high speed, then this hidden sword technique transfers all of the stored inertia from the user and weapon movements to the point of contact with the target; with Orochimaru, the secret weapon of the Chiba Family, the stored inertia could be further enhanced by closing in from a distance further out to reach a maximum of 10 tons.
- Yamatsunami Tsubamegaeshi (山津波・燕返し)
- A variant technique of Yamatsunami, where inertia is not restored upon the downwards swing of the sword, but rather upon the slice upwards. The weight of the blow is the same as the main technique where as it's speed is slower.
- Kirikage is a secret blade technique of the Chiba Family, where a magic sword causes damage to the Psion body of another. A Non-Systematic magic that when slashed at the "soul" of a person that bypasses physical armor like the USNA Thrust Suit, it deprives them of their motor functions.
- Non-systemic magic, which is just like 'Gram Demolition' ands it's classified as counter-magic. It neutralises magic by 'cutting' the magic sequences in the information dimension with a psionic blade. Tatsuya tells her to use her intuition to know the position of the enemy's magic to send a magic that is a Psion Sword. 
- Erika uses a CAD shaped as a Telescopic Steel Baton (Nightstick) as her primary weapon. It has an Activation Sequence engraved upon it by the Isori Family (Isori Kei).
- The secret weapon, Orochimaru (大蛇丸), is the blade forged to use Yamatsunami. The entire length of the sword is 180 cm, the blade alone being 140 cm. It is one of the weapons that is in the pinnacle of sword-type armaments forged by the Chiba Family.
- Mizuchimaru (ミズチ丸) is a weaponized-integrated CAD in the shape of a sword. It was made by the Isori Family and adjusted by Tatsuya to be a downsized version of Orochimaru, allowing Erika to use a weaker version of Yamatsunami. During Erika's battle against Canopus, Mizuchimaru is broken with Molecular Divider but Erika is still able to use Kirikage with the broken sword that incapacitates Ralph Algol. 
- One of Erika's weapons is probably named after a Japanese folk tale character called Orochimaru (大蛇丸). 
- Volume 1, Chapter 1
- Volume 9, Chapter 5
- Volume 3, Chapter 4
- Volume 1, Chapter 5
- Volume 4, Chapter 10
- Volume 10, Chapter 10
- Volume 21, Chapter 3
- Volume 13.5, Chapter Summary
- Volume 7, Chapter 11
- The Irregular at Magic High School The Movie: The Girl Who Summons the Stars
- Volume 1, Chapter 2
- Volume 7, Chapter 12
- Volume 11.6, Chapter 18
- Volume 31, Chapter 4
- Volume 4, Chapter 12
- Volume 9, Chapter 6