Mairu Orihara

Mairu Orihara (折原 舞流 Orihara Mairu) is one of Izaya Orihara's younger sisters and the twin sister of Kururi Orihara. Like her brother, Mairu is a troublemaker at school and introduces herself as liking to read encyclopedias, manga, and adult magazines. She also declares that she is bisexual and is looking for girls who are interested. Along with her sister, she is a big fan of Kasuka Heiwajima, stating that she would gladly see Izaya die in front of a truck for a chance to meet him.

Appearance
Mairu has long brown hair that is often tied back in a braid, brown eyes and pink square-rimmed glasses. Her school attire consists of a black sailor uniform with a long skirt, which contrasts with the Raira Academy uniform and, combined with her glasses, gives her the look of a stereotypical bookworm, contrasting with her outgoing nature. Her street attire generally consists of a yellow hoodie with dog ears and a green skirt.

Kururi Orihara usually wears a similar outfit with an opposite color scheme.

Personality
Mairu is a extremely energetic and outgoing girl who is a textbook chatterbox in real life. She can also be rather vulgar and usually has no problem going into extreme detail about subjects about sex and pornography even when others are listening, which usually prompts Kururi Orihara to calm her down. While usually acting playful in most situations, Mairu has been shown to get serious if there is genuine danger, especially if it might pose a threat to Kururi. Online, her personality is almost completely opposite. Mairu usually only replies with short phrases or a single word much like Kururi does in real life. However, her vulgarity still shines through, and she occasionally gets blocked for obscene content.

Like Kururi, Mairu regularly attends martial arts classes and is a skilled fighter. She uses her fighting skills more frequently than her sister.

Background
For the most part, the Orihara twins and Izaya Orihara shared a fairly normal childhood. Their parents frequently worked overseas and Izaya claims that he practically raised the twins himself. At some point when the twins were young, he asked the twins the point of living when everything they do is the same. Shinra Kishitani believes Izaya did not mean the twins harm but only wanted to see what they would do. Instead of feeling discouraged or fighting, Mairu and Kururi tried to compensate for each other's shortcomings by rolling dice to decide who would take on which personality traits and in this way embody all of the positive traits of humankind. Izaya refers to the two as monsters and claims responsibility for their peculiar natures.

Hollywood Arc
Shortly after the opening ceremonies end, Mairu wastes no time in making a very odd and somewhat negative first impression on her class. In her introduction, she freely admits to being bisexual but is more interested in girls, and proceeds to openly read a pornographic magazine in the middle of class. Not surprisingly, several of the girls decide to pick on Mairu and the next day, she arrives to find her desk completely vandalized complete with scratches and poorly written graffiti. Still wearing a bright smile, she demands to know the vandal's identity. None of the students answer but the snickering of the girls near the front makes it obvious who it was. Mairu grabs a box of thumbtacks from the back of the classroom and leaps from a tall cabinet across the room on desktops to reach the bullies. She grabs one of them at random and slams her down on the floor, threatening to pour the tacks down her throat if she didn't reveal who destroyed her desk. The girl points to her friend. Mairu throws the thumb tacks with amazing accuracy at the door handle, freezing the bully. Mairu grabs her shoulder and drags her outside and into the girls' bathroom expressing a sudden desire to "get to know her." The girl is not seen for the rest of the day.

After Mairu hears about how Aoba Kuronuma stopped the bullies from picking on Kururi Orihara, she and her sister show their "thanks" by kissing Aoba on the lips, though Mairu appeared to follow suit only for the sake of Kururi's residual kiss. She warns Aoba that Kururi is actually more aggressive than she is despite how she acts and gives him a warning about falling in love with her.

At some point, the twins receive a link from Namie Yagiri to join the chat room.

After seeing Celty Sturluson in action against the cops on the news, Mairu and Kururi decide to try and meet the Black Rider. They go on an adventure throughout Ikebukuro to find her. They investigate the area shown on the news but only find an envelope with 1 million yen in cash contained inside. They also run into a heavily injured Egor who politely asks the two of them for directions to a restaurant called Russia Sushi. After getting the directions from Shizuo Heiwajima, they place Egor in Simon Brezhnev and Denis' care. They call Shinra Kishitani over (who had just finished treating Ruri Hijiribe) but he charges over 200,000 yen for the sudden interruption of a meeting with one of his favorite celebrities. With no one else to turn to, Mairu and Kururi agree to pay the 200,000 with the money they found. They also give Denis the remaining 800,000 yen to pay for him to hire Celty to transport an unconscious Egor for a day, effectively returning the 1 million yen Celty lost.

The next night, the two of them were harassed by a few Toramaru members when they were asking about the Headless Rider. Kyouhei Kadota notices them and chases the Toramaru members off. Kyouhei agrees to drive them to a safer place and calls Saburo Togusa to pick them up. As soon as they get in the van, the Toramaru members come back and chase them around the city. They also pick up Mikado Ryuugamine, Anri Sonohara and Aoba along the way with Aoba expressing shock at the two of them being in the van. After Celty joins up with them, they express wonder and delight at having finally found the Black Rider. Later, when Celty decides to hold off the Toramaru members with her shadow, Mairu, Kururi, Aoba, Mikado and Anri are dropped off near a police station but they all run back to the overpass to see what is happening. They watch in awe as Ruri and Egor beat up the Toramaru members. Later, they attend the hotpot party held at Shinra and Celty's apartment.

Daily Life Arc
Mairu and Kururi Orihara were hired by Namie Yagiri to spy on Mika Harima and Seiji Yagiri and report back to her on their activities. After giving a very detailed report on their most recent date, the twins accept their payment and leave, seeing that Namie was becoming noticeably more infuriated as they continued their description.

The next day, the two of them make friends with Akane Awakusu, who had just joined their martial arts classes, and decide to hang out once practice was over. Along the way, they manage to run into Shizuo Heiwajima, Tom Tanaka and Vorona who were in the middle of work. Mairu takes an instant liking to Vorona and tries to hug her, but Vorona's quick reflexes keep Mairu at bay. Tom and Shizuo decide to tackle the next house they need to visit on their own, leaving the girls to chat amongst themselves. As Vorona questions the twins on their relationship to Shizuo, several thugs arrive and attempt to kidnap them to use as hostages against the aforementioned debt collector. The girls make short work of the thugs, forcing them to run away as Shizuo and Tom return. Shortly after the dust settles, Mairu and her sister return home.

Adabashi Arc
With Taro Tanaka and Kanra gone from the chatroom, Mairu and Kururi Orihara decide to try and invite new members to join which include Gaki (Mizuki Akabayashi), Pure water 100% (Aoba Kuronuma), Chrome (Izaya Orihara), Shoro (Eijirou Sharaku) and Saki (Saki Mikajima).

Dragon Zombie Arc
Izaya Orihara passes by Mairu and Kururi Orihara's martial arts school where Izaya is told to die by Mairu. Izaya sarcastically expresses that his feelings are hurt only to be attacked by Eijirou Sharaku (the Orihara twins' martial arts instructor). The two of them cheer Eijirou on until the fight is interrupted by their other instructor, Mikage Sharaku which allows Izaya to escape. Eijriou laments at having lost Izaya again and also asks if Mikage still had feelings for Izaya despite what he did to her in high school. Mairu and Kururi wait with anticipation for Mikage's answer but Mikage only proceeds to attack Ejirou relentlessly for several hours.

Later in the novel, Mairu is on her way to meet up with Kururi but is being stalked by several members of "Heaven's Slave." The group was intending to kidnap Mairu to hold hostage against Izaya but that plan is interrupted by Eijirou who beats up all of their members. Mairu realizes that Kururi may also have been attacked and calls to discover that Celty Sturluson protected her under Izaya's orders.

The twins reunited and encounter Izaya alone in the street. They thank Izaya for protecting them and state that when Shizuo finally kills Izaya, they'll cry a little before they laugh about it. The siblings then walk back home from there.

Durarara!! SHx2
Mairu and Kururi Orihara, along with Aoba Kuronuma, Akane Awakusu and two freshmen: Yahiro Mizuchi and Kuon Kotonami, are on a hunt to find the Headless Rider to prove whether or not she is really behind the sudden rise in missing persons. When she and Kururi take Yahiro and Kuon to their gym, Mikage notices the scars on Yahiro's hands. She tells Mairu that he must have gotten those scars from smashing teeth and they wonder how many teeth he had smashed in to get those scars.

Trivia

 * The name Mairu means "dance" (舞) (mai) and "flow" (流) (ru).
 * Mairu's surname Orihara means "occasionally" (折) (ori) and "field, plain" (原) (hara).
 * Mairu is the first person to be censored on the Chat Room for mature content.
 * In the first season of the English dubbed version of the anime, Mairu and Kururi Orihara share the same voice actress.
 * In the second season, they each have a different English voice actress.
 * Mairu is a competent martial artist who fights directly using kicks and punches (unless she feels like using other weaponry like thumbtacks) while her sister, Kururi, defends herself using items such as pepper spray, stun guns, or other items in her bag.
 * The twins' birthday is on Valentine's Day
 * The twins' and Izaya Orihara's maternal grandparents are deceased.
 * The twins appeared briefly in the background in the 8th episode.