When Haruyuki first meets Yuniko, she has the personality of a normal girl. She was sweet and caring to Haru. She presented good manners such as greeting him and was shown to be able to cook well.
However, after Haruyuki discovered Yuniko's identity and that she was the current Red King, her personality changed radically, showing her true colors. She is rude and gets frustrated easily when things don't go her way. Yuniko has a rivalry between Kuroyukihime, and the two get on each other's nerves, but they also care for each other more than they let on.
In spite of her polite personality being a facade, Yuniko is also somewhat kinder than she realizes. While she claims that bonds between Burst Linkers are meaningless, Kuroyukihime points out that Yuniko still wants to believe in the "ultimate bond" between a "parent" and "child," which is why she hesitates to use Judgment Blow on Chrome Disaster. She also will sometimes help out Haruyuki and the others while claiming that it's only repaying a debt or for her own legion's interests.
In battle, Yuniko appeared to be sadistic, enjoying to see her enemies being destroyed by her full barrage of heavy weapons.
Yuniko is an elementary student and attends a private school. Her duel avatar is Scarlet Rain who is the current Red King following Red Rider. Her reasoning behind finding Haru was to ask for his help to stop the 5th Chrome Disaster which is currently one of Yuniko's legion members, Cherry Rook.
Yuniko's appearance is that of a small girl with red eyes. She has red hair and is made into two pigtails with two blue bowties. When she is first seen in the series in an attempt to get closer to Haru, she is seen wearing a white blouse with a blue string tie. On top, she wears a pink apron with the word "cat" on it with a picture of one and a fish.
Her standard attire, however, consists of a long-sleeved red and black striped shirt with ripped short jeans and black stockings. She still retains her two blue bowties in her pigtails.
Yuniko infiltrated Haruyuki's house by disguising herself as his second cousin, Tomoko Saito. She later unexpectedly tried joining him in the bathroom. However, Haru was reviewing his family photos and noticed the tan mark of a Neuro Linker on her neck, contrary to her claim that she doesn't use one, and suspected her of being a Burst Linker. Deciding to drop the act, Yuniko revealed her identity as Scarlet Rain. As she turned to leave, she slipped on a towel and fell into the bathtub as Haru caught her. Noticing that his hands were touching her upper chest, Yuniko vowed to kill him and retrieved her Neuro Linker. By the time she returned, Haru, having just got off the phone with Kuroyukihime, who realized she was the Red King. With a devious look on her face, she challenges him to a match and defeats him. With a gun pointed towards the head of Silver Crow, she requested a real-life meeting with his legion master, Black Lotus.
Takumu Mayuzumi theorizes that knowing Haru, after a few days living with Yuniko, he would've grown fond of her and then she'd claim to be a new Burst Linker being bullied by older members of her legion and ask for his help. If Haru left for the Red Legion, Prominence, Yuniko would've threatened him with the Judgement Blow to force him to do her bidding.
Upon setting up the meeting, the Nega Nebulous inquired how Yuniko learned Haru's identity. She admits that she had gotten a pass to tour the schools of Suganami ward, which would've given her access to the names of Umesato Junior High students. She waited at a cafe, watching students exit the school and were surprised to see Silver Crow logged onto the Global Net and Haru was that person. She reveals that she sought him out because she needs his avatar to bring down the 5th Chrome Disaster. Kuroyukihime took offense, claiming to have witnessed the destruction of the Armor of Catastrophe. Yuniko contradicted that claim, stating that the new Chrome Disaster is attacking her legion, originating with her parent, Cherry Rook. As Haru expressed confusion, both kings decided to show him what they are talking about by showing a record of the battle with previous Chrome Disaster. After explaining the nature of Chrome Disaster, Yuniko explains that she needs Haru because the monster can leap great distances and his flight ability can help catch him so she can use the Judgement Blow. In return, she will tell her legion to agree to a temporary truce with the Nega Nebulous.
After Taku left, Yuniko's teasing towards Haru further annoyed Kuroyukihime, who chose to stay the night rather than leave her boyfriend alone with her. Together, they went out shopping for food. While at a supermarket, Yuniko guessed that Haru was daydreaming about her, wondering if she wanted to be spoiled by good parents. She tells him that due to the accelerated technology of the game, real life fades away for her. If he found out how much time she and Kuroyukihime spent in Accel World, it would blow him away. Haru asks how long did she spend there, to which she refused to answer.
After having dinner, Yuniko bathed with Kuroyukihime, and after getting out, she attempted to comment on Kuroyukihime's nude figure but was punched in the stomach in mid-sentence. She and Kuroyukihime then slept peacefully in each other's arms on Haru's couch.
The next morning, Yuniko sees both Haru and Kuroyukihime off to school. But then she noticed the boy daydreaming and pinched him by the cheek, accusing him of having some creepy fantasy. She then spends the entire day at Haru's apartment before he came home. He then asked her if she resents Kuroyukihime for killing Red Rider, the previous Red King. Yuniko says that she never even met him and is glad that Kuroyukihime got rid of him so she could take his place. She then explains the Kings' theory that once someone reaches Level 10, then the game would end. Feeling embarrassed, she kicked at Haru, but he caught her foot, telling her how he experienced a game that never ends, resulting in his fellow players becoming bored and moving onto other games. Eventually, even the NPCs disappear. To that end, he wishes to find out the meaning of Brain Burst even if it ends the game. Yuniko complements him, but then angrily kicks him for holding her foot for a prolonged period, calling him a pervert.
Once Kuroyukihime and Taku, the foursome accelerate into the Unlimited Field, where they intended to catch Chrome Disaster. They explained the purpose of the Unlimited Field to Haru, who figured that since there's no time limit, a day in the real world would be three years in this one. He became excited, knowing he could do all sorts of things but the other advises him not too, as spending a prolonged period in the Unlimited Field would change him.
Yuniko then got their mission back on track, reminding that they got to catch the train Chrome Disaster is on. Haru/Silver Crow is quickly volunteered to fly them there, and Scarlet Rain is quick to ask that he hold her, causing him distress. Black Lotus takes offense, stating that Crow will be the one to carry her since she has only blades for limbs. Rain criticized Lotus for her flawed avatar. Taku/Cyan Pile attempts to get a word in, but both kings shout him down. Haru then chose to carry both kings in his arms and Cyan Pile on his legs, though his speed was reduced considerably. As they make their way across the city, the other explained to Haru how Burst Linkers fight Enemies and other Burst Linkers in the Unlimited Field in hopes of leveling up.
Suddenly, long-ranged fire forces them down into a crater, where they are surrounded by Crypt Cosmic Circus, led by the Yellow King himself, Yellow Radio, who revealed that he was there to claim Scarlet Rain's head in compensation for some of his members having Brain Burst uninstalled, and deflected any accusations that he was responsible for Cherry Rook becoming the Chrome Disaster.
A battle ensues between the Yellow King's forces and Yuniko's group until the former group flees from the Chrome Disaster. During the battle with Chrome Disaster, Yuniko attacks Kuroyukihime and, claiming that bonds between Burst Linkers are meaningless, and demands that Haruyuki and Kuroyukihime leave. Kuroyukihime, however, realizes that Yuniko does not truly mean what she says, and insists that Haruyuki helped her since she's their friend. Yuniko tries to finish Chrome Disaster off with Judgement Blow, but is unable to do so, and is saved by Haruyuki, who proceeds to defeat Chrome Disaster. After performing Judgement Blow on Cherry Rook, Yuniko reveals their relationship and blames herself for him choosing to take on the armor. She then has everyone present confirm that they did not receive the armor.
Sometime later, Yuniko manipulates her way into Haruyuki's apartment again by pretending to be the daughter of one of his mother's coworkers. She also reveals that she was able to clear herself, although she's frustrated that she has no evidence to implicate Yellow Radio. She then says that Cherry Rook's desperation was caused by his being taken by relatives and having to move, but says he's back to his old self, and she hopes to keep in touch with him while playing other VR games.
Haruyuki seeks Yuniko to help him teach Takumu about the incarnate system. Upon hearing this, Yuniko's expression changes to anger. Adjourning to a private room in the cake shop they are in, she starts yelling at him for talking about incarnation in a public place. She then calmed down and listened to the duo's story of how Seiji Noumi/Dusk Taker is terrorizing them. She finds it no concern of hers, as the Nega Nebulous is not her legion, but before they can say anything, she guesses that Haru would remind her that they helped Prominence, so she agrees to teach Taku the incarnate system. They then use the unlimited burst command to go to the Unlimited Neutral Field. After arriving, she demonstrates incarnate skills and she reveals that she only knows two of the four skills. Yuniko stays to train Takumu while Haruyuki leaves with Blood Leopard to investigate on how certain Burst Linkers don't appear on the matching list even though they are connected to the network.
In the anime, after Haru finally manages to destroy Dusk Taker, he, Kuroyukihime, Yuniko, and Leopard meet to discuss how Noumi lost his memory of Accel World. Kuroyukihime says they need to ask the creator of Brain Burst to find out. In the light novels this did not occur, instead it is implied that the Nega Nebulas members only passed on to Yuniko and Pard that the situation with Dusk Taker had been resolved.
Though Scarlet Rain does not appear, Pard passes on a message from the Red King to the Nega Nebulas members participating in the Hermes Cord race, and it is implied that she was watching the race from one of the grand stands as the Acceleration Research Society interrupted the race, also bearing witness as Haruyuki temporarily transformed into the new Chrome Disaster.
Because of the events that occurred during the Hermes Cord race, Scarlet Rain joined the other Kings in a meeting to discuss the Research Society, the revelation about the incarnate system to all Burst Linkers, and Silver Crow's parasitization by the Armor of Catastrophe. During the meeting, Yuniko trades insults with Yellow Radio regarding her impure color, while Haru is concerned about Niko's reaction to him becoming the sixth Chrome Disaster, since her use of the Judgement Blow on Cherry Rook proved pointless in removing the Armor from the Accelerated World. Indeed, he begins to fear she hates him once Yuniko interrupts the discussion to prioritise the matter regarding Haru and the Armor of Catastrophe. By the meeting's end, it is decided that Silver Crow will have a week to try and get rid of the Armor or face being hunted by all the other Legions.
Immediately after the meeting, Yuniko goes to visit Haru in the real world to explain her actions and that she holds no grudge against him, having only brought up the Armor to prevent Yellow Radio from taking point on the issue. During their conversation, Yuniko admits to fearing the other Kings due to the information pressure caused by their high level and long-time experience within Brain Burst. Uncertain that she could defeat any of them in a duel, and with the recent confirmation of the rumors that former Burst Linkers lose all their memories of the Accelerated World, Yuniko makes a pact with Haru to remain in touch with each other even if one of them loses Brain Burst.
Yuniko's avatar resembles that of a prince. Her pigtails are longer, and she now wears two white bowties instead of blue ones. Her clothes are that of a prince's suit with most of it being red. She wears a blue cape, white gloves and brown boots with a sword attached to her waist.
Yuniko's Brain Burst character is Scarlet Rain, who is level 9, and leads the legion Prominence as the second and current Red King. Scarlet Rain closely resembles Yuniko's real appearance such as her small body and two pigtails. Her body is metallic, has green eyes and is mostly colored in red with some parts pink. She also carries a hand blaster, called Peace Maker, with her which is usually attached at her waist.
She also knows of the Incarnate System and has a few incarnate skills, which she demonstrates while teaching Silver Crow and Cyan Pile.
- Peace Maker – Scarlet Rain's starter equipment. It is her primary weapon when the Invincible is inactive. It is a handheld blaster which fires a limited number of physical bullets. Its shots can be charged for a stronger attack, and she claims that it can open a hole in the ground given 20 seconds of charging.
- Invincible – The result of Scarlet Rain taking all of her level-up bonuses and pouring them into her firepower and range, the Invincible is a gigantic artillery platform with a large array of weapons and high defenses, earning her the nickname "Immobile Fortress". It is made from 5 Enhanced Armaments.
- Laser cannons (unnamed) – Typically referred to simply as "the Invincible's laser cannons", these are two laser cannons on the sides of the Invincible. They require 1 second to charge up, and cannot fire continuous beams.
- Missile racks (unnamed)
- Thruster blocks (unnamed) – While the Invincible gave Scarlet Rain the title "Immobile Fortress", it is not actually immobile. The Invincible's back-mounted rocket thrusters allows it to perform quick dashes.
- Cockpit (unnamed)
- Legs (unnamed)
- Dreadnought – Also using the 5 components above, the Invincible can be reconfigured into the Dreadnought with the voice command "Change: Dreadnought". The Dreadnought is a heavily-armed, twelve-wheeled truck almost 3 meters tall. In this mode, the Invincible's legs form the bottom of the truck, the gun blocks form the front, with the missile launchers behind them, and the rocket thrusters form the back. The laser cannons attach to the sides. Missile racks can open up on its roof Its firepower is not as high as the Invincible proper. It has speakers so Scarlet Rain can communicate with those on top. It also has no brakes.
- Vision Extension – This ability grants her information such as thermal maps and wind direction. If she concentrates hard enough, she can also scan other Burst Linkers to determine their information pressure.
- Hailstorm Domination – While equipping her huge armament 'Invincible', it'll fire off an entire fortress battalion of missiles all over the field with enough firepower to wipe out an army on the ground or any fliers in the air.
- Heat Blast Saturation – This Special Move utilizes the laser cannons of the Invincible, firing two lasers that merge and carry on as an enormous light beam. Once it hits a target, it expands into a ball of light, then explodes.
- Coronal Mass Ejection – A special move that involves firing a barrage of missiles while constantly firing 4 gatling guns and her laser cannons. This requires the Invincible.
- Judgement Blow – As the Legion Master of Prominence, Scarlet Rain is able to channel the Legion Master's ability to instantly uninstall Brain Burst from any of her legion members, given that she is within range.
- Radiant Beat – Allows Scarlet Rain to form incarnate flames around her fist and launch them at her enemies as a fireball. This has a physical impact as well, acting like a punch.
- Radiant Burst – The multiple-shot version of Radiant Beat, released by punching with both hands repeatedly.
- Pyro Planing – The second of her Incarnate abilities, this allows Scarlet Rain to move at extreme speeds by melting the ground and skating upon it. Niko later reveals this incarnate ability is born from her fears of being close to others so that she can always run away. This ability also allows her to climb walls, albeit with not a very high success rate.
- Scarlet Exploder – This Incarnate skill fires a red bullet of light out of her Peace Maker.
- In the light novel, she is said to have green eyes.
- Rina Hidaka, the Japanese voice actress for Yuniko, also voices Silica (Ayano Keiko) from Sword Art Online.
- Sarah Anne Williams, the English voice actress for Yuniko, also voices Lisbeth (Shinozaki Rika) from Sword Art Online.
- Accel World Episode 08
- Accel World Episode 09
- Accel World Volume 11 Chapter 4
- Accel World Volume 21 Chapter 3
- Accel World Volume 16 Chapter 2
- Accel World Volume 16 Chapter 4
- Accel World Volume 13 Chapter 9
- Accel World Volume 14 Chapter 1
- Accel World Volume 12 colored pages
- Accel World Volume 11 Chapter 5
- Accel World Volume 14 Chapter 2
- Accel World Volume 21 Chapter 6
- Accel World Volume 15 Chapter 9