Yui Kodai is a student in Class 1-B of the U.A. High School Hero Course in the series My Hero Academia, known for her quiet demeanor, poker face, and size-altering Quirk that specializes in support combat.
Name: Yui Kodai
Hero Name: Rule
Gender: Female
School: U.A. High School, Hero Course
Class: 1-B, Student No. 6
Birthday: December 19
Height: 160 cm
Blood Type: B
Place of Birth: Shimane Prefecture
Favorite Things: Matryoshka dolls, tomatoes
Quirk: Size
Voice Actor (anime): Ibu Yulian
Yui Kodai is a support-oriented hero-in-training whose Quirk allows her to freely change the size of inanimate objects.
She is considered one of the most beautiful girls in the Hero Course and even had an unnoticed fan club in middle school.
Yui has black bobbed hair and a perpetually blank expression, rarely speaking more than a few sounds.
Her most common utterances are simple syllables like “n,” “nn,” or “nnn!” which, despite their simplicity, often convey her intent to those around her.
She is taciturn and extremely reserved, but not gloomy or unfriendly.
Compared to many of the bright and outgoing girls at U.A., Yui stands out as quiet, calm, and low-key.
Her composure suggests she has nerves of steel, and she rarely seems flustered even in tense situations.
However, she can show small, subtle reactions, such as letting out an enthusiastic “nnn!” when eating tomatoes, which she loves.
She is notably uncomfortable around Neito Monoma, and treats him with particularly cold indifference.
Despite being very popular—especially with boys—she is either unaware of or unconcerned with that popularity.
During middle school, a fan club formed around her without her knowledge.
She graduated without ever realizing she had that fan following.
Yui’s defining physical trait is her straight black bob haircut and expressionless face.
Her stoic look contributes to her “ice-cool beauty” image and fuels her popularity.
Her hero costume reflects her size-based support Quirk and evokes the image of a giant hero of light.
It is a red-and-white bodysuit with a short white mini-skirt, visually reminiscent of classic giant heroes from special-effects shows.
She wears a sun visor with a crest-like fin on top, enhancing the “giant hero” motif.
For utility, she has storage pouches on both shoulders and around her waist.
These pouches allow her to carry various items she can enlarge or shrink with her Quirk.
Combined with her calm demeanor, the costume reinforces her role as a strategic, support-focused hero.
Size
Yui’s Quirk, Size, allows her to freely change the size of inanimate objects she touches.
Living things are excluded, so she cannot resize people, animals, or other biological beings.
She can shrink large items to make them portable or enlarge small objects into obstacles, platforms, or weapons.
For example, she can turn something as small as a nut into something human-sized or even bigger.
The effect is similar in operation to Ochaco Uraraka’s style of canceling her Quirk.
By putting her hands together, Yui can cancel the size changes and return objects to their original size.
This Quirk is highly versatile and tricky, lending itself well to support and battlefield control.
In combination with other Quirks, it can create long-range, wide-area attacks or improvised defenses.
During the joint training battle between Class 1-A and Class 1-B, she combined her Quirk with Nirengeki Shoda and Reiko Yanagi for a long-distance, indiscriminate attack strategy.
Her ability to manipulate the size of projectiles and tools makes her a valuable teammate in multi-person operations.
U.A. Sports Festival
In the Sports Festival’s first event, the Obstacle Course, Yui placed 31st and successfully cleared the stage.
In the second event, the Cavalry Battle, she teamed up with Tetsutetsu Tetsutetsu, Manga Fukidashi, and Kojiro Bondo but was ultimately defeated and did not advance.
Training Camp and Test of Courage
At the Forest Training Camp, Yui participated in the class-versus-class Test of Courage.
In this event, she worked with Juzo Honenuki’s Quirk, being buried in the ground with only her face exposed to scare passing students.
When the pair of Kyoka Jiro and Toru Hagakure came by, an accident occurred.
Startled, Kyoka Jiro reflexively jabbed her Earphone Jack into Yui’s eye area, causing a brief panic.
Fortunately, the incident did not result in serious injury.
Yui emerged unharmed, despite the unfortunate nature of the scare attempt.
Joint Training: Class 1-A vs Class 1-B
In the joint training battle between Class 1-A and Class 1-B, Yui appears in the fifth match.
The participants are Izuku Midoriya, Ochaco Uraraka, Mina Ashido, and Minoru Mineta from Class 1-A versus Neito Monoma, Yui Kodai, Nirengeki Shoda, Reiko Yanagi, and Hitoshi Shinso representing Class 1-B.
At the start of the match, the fight splits into a three-on-three: Ochaco, Mina, and Mineta versus Yui, Shoda, and Yanagi.
These three from Class 1-B display strong coordination, using their Quirks in combination for tactical attacks.
Using her Size Quirk together with Shoda’s and Yanagi’s abilities, Yui helps orchestrate long-range, area-covering assaults.
Her role emphasizes support and battlefield control rather than direct confrontation.
After the brief interruption involving Izuku Midoriya’s power, the match resumes.
While Mina and Mineta are being engaged, Ochaco takes advantage of an opening and manages to restrain Yui and Yanagi in quick succession.
Despite their strong teamwork, the fifth match ends in victory for Class 1-A.
Yui and her teammates are captured, showcasing both their potential and the limitations of their strategy.
Final Battle: Heroes vs Villains
In the final large-scale conflict between the heroes and the Paranormal Liberation Front, Yui joins the heroes’ side.
She participates in the operation to recapture Gigantomachia, targeting the gigantic villain.
In this mission, she fights alongside Juzo Honenuki, Reiko Yanagi, Class 1-A’s Mina Ashido, Eijiro Kirishima, Minoru Mineta, and Hitoshi Shinso, who is scheduled to transfer into the Hero Course.
Yui contributes with her size manipulation to support the team and disrupt the enemy’s movements.
Yui’s hero tagline describes her as a “size-changing support specialist who can freely alter object dimensions”.
She is also promoted as a “poker-faced beauty who communicates almost entirely with ‘n’ sounds”.
Her love of tomatoes is so strong that taking a bite can make her exclaim “nnn!” with unusual intensity by her standards.
Her fondness for matryoshka dolls fits thematically with her size-based Quirk, as they are nested objects that visually play with scale.
Among the students of the U.A. Hero Course, Yui is said to be the most popular girl.
Her quiet nature, cool aura, and striking looks contribute heavily to this reputation.
In a spin-off light novel focusing on U.A. student life, she has a role in the cultural festival story.
She uses her Quirk behind the scenes to move stage sets for a play titled “Romeo and Juliet and the Prisoner of Azkaban – The Return of the King.”
In that performance, Yui also plays the heroine, Juliet.
The last part of the story reveals a surprising twist about her role, adding an extra layer of charm to her usually understated presence.
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