Mairu Orihara

"Nice to meet you! My favorite books are the encyclopedia, manga, and porn mags! I go for both teams when it comes to love and lust! But the spot in my bed for men is already spoken for, so don't even try! I can go out with as many girls as I want, however, so keep that in mind when you propose a relationship!"

- Mairu Orihara, Durarara!!x4

Mairu Orihara (折原 舞流 Orihara Mairu) is one of Izaya Orihara's younger sisters and the younger twin sister of Kururi Orihara. Like her brother, Mairu is a troublemaker at school and introduces herself as an avid reader of encyclopedias, manga, and adult magazines. She also declares that she is bisexual and is looking for girls who are interested. She and her sister are enthusiastic fans of Kasuka Heiwajima, and Mairu has declared she would gladly see Izaya die if it would allow her a chance to meet her celebrity crush.

Appearance
Mairu has long brown hair that is often tied back in a braid, brown eyes, and pink square-rimmed glasses. She typically wears a long-skirted black sailor outfit to school rather than the standard Raira Academy uniform; her attire and her glasses together give her the look of a stereotypical bookworm—a 'look' that contrasts with her vivacious nature.

Her usual streetwear consists of a dog-eared white hoodie and a black skirt (yellow and green respectively in the anime), the outfit similar albeit possessing an opposite color scheme to Kururi's own usual streetwear.

Personality
Mairu is an energetic, outgoing chatterbox who has few compunctions about vulgarity or talking about sex-related subjects in overt detail. For all that she acts generally playful, she will treat genuine danger seriously—especially if said danger might pose a threat to Kururi. Her 'online personality', meanwhile, echoes Kururi's 'real life personality'; she communicates in reticent one-word or few-word sentences while Kururi plays the chatterbox. However, Mairu's vulgar tendencies persist even when online, leading her to be occasionally blocked from chat rooms for obscene content.

Like Kururi, Mairu regularly attends martial arts classes and is a skilled fighter. She makes use of her combat abilities more often than her sister does.

Background
The Orihara twins' childhood is—for the most part—a fairly normal one, though Izaya claims to have 'practically raised' them due to their parents frequently working overseas.

On one occasion, when the twins are still young, Izaya asks Mairu and Kururi what is the point of them living when they live their lives the same way. Rather than lashing out or becoming discouraged, Mairu and Kururi attempt to compensate for each other's shortcomings by rolling dice to decide their respective personality traits; in this manner, they hope to embody all of humankind's positive traits. Izaya subsequently deems them 'monsters' and claims responsibility for their peculiar natures.

Hollywood Arc
Shortly after Raira Academy's opening ceremonies end, Mairu introduces herself to her class and proceeds to openly read a pornographic magazine in the middle of a lesson. Several of her female classmates take umbrage with her behavior; on the morning of her third day at school, Mairu arrives to find her desk vandalized with graffiti and scratches. She demands the vandal's identity with a smile and, though no one answers, the girls' snickering identifies them all the same.

Mairu seizes a box of thumbtacks, dashes forward to slam one of the girls onto the floor, and threatens to pour the tacks down her throat if she does not name the vandal. The girl points to her responsible friend, and Mairu hurls the thumbtacks at the vandal to stop her in her tracks; she then drags the girl to the girls' bathroom to "get to know her" better, and the girl is absent for the rest of the school day.

After Mairu learns that Aoba Kuronuma stopped the bullies from targeting Kururi, she and her sister express their "thanks" by kissing Aoba on the lips—though Mairu appears 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 Kururi acts, meaning that Aoba should think twice about falling in love with her.

At some point, the twins receive a invite link to the Chat Room from Namie Yagiri.

After seeing Celty Sturluson facing off against the cops on the news, Mairu and Kururi decide they want to meet the Black Rider and wend their way through Ikebukuro's streets in an attempt to locate her. When they reach the location where Celty was seen on the news, they happen to stumble across an envelope containing one million yen in cash. They also stumble open a heavily injured Egor who politely asks them for directions to a restaurant called Russia Sushi.

Upon obtaining directions from Shizuo Heiwajima and arriving at the aforementioned establishment late at night, the twins entrust Egor to Russia Sushi employees Simon Brezhnev and Denis and telephone Shinra Kishitani for emergency medical assistance. Shinra charges them over 200,000 yen for having called him away from treating Ruri Hijiribe—one of his favorite celebrities—and Mairu and Kururi pay the asked sum via the envelope they found; they then give Denis the remaining 800,000 yen in exchange for hiring Celty to transport an unconscious Egor for a day—unwittingly returning to Celty the one million yen she lost.

The afternoon of the next day finds Mairu and Kururi wandering the streets after class. Where the weather is all sunshine, Mairu is all gloom over a new tabloid article speculating that Kasuka and Ruri are having a tryst since the two were spotted meeting overnight. As she laments the loss of Kasuka's heart to someone else, Kururi tries to comfort her with a kiss. She comforts Mairu successfully, but Mairu's newfound cheer is muted shortly thereafter when a four Toramaru bikers swagger up and harass the twins for information on the Headless Rider's whereabouts.

Kyouhei Kadota comes to the twins' rescue by headbutting one of the four men, giving Kururi the opportunity to kick another man in his genitals and Mairu to kick the third man in his jaw. As Erika Karisawa and Walker Yumasaki tie up the first man, the last man standing retreats on his bike. Kyouhei introduces himself to the twins and recognizes them as Izaya's sisters; though he attempts to persuade them to go hunker down at home, Mairu adamantly insists that she and her sister have to continue wandering around Ikebukuro until they receive a certain telephone call.

Kyouhei compromises by suggesting the twins tag along with Erika and Walker when the latter pair are due to guide some other Raira students around town later today. The twins agree and, after Kyouhei calls Saburo Togusa, wait with Kyouhei, Erika, and Walker at a nearby café for Togusa to arrive in the van. When Saburo does arrive, so to does a Toramaru group five times the size of the first one in a bid for revenge—and they proceed to have a car chase with Saburo and company around the city.

Saburo briefly stops to pick up Mikado Ryuugamine, Anri Sonohara, and Aoba along the way, and Aoba expresses shock at the sight of the twins inside the van. When Celty herself catches up with the van, the twins express their own shock and delight at having finally encountered the Black Rider. Celty eventually decides to personally fend off the Toramaru members at an overpass so that the van may finally drive to freedom; Saburo thence drops off Mairu, Kururi, Aoba, Mikado, and Anri at a nearby police station—but the Raira students all run back to the overpass anyway to see what is happening. Upon their return they find Egor and the serial killer Hollywood jointly assaulting the Toramaru members, and all present watch the one-sided fight in awe. Later, Mairu and Kururi attend a hotpot party hosted at Shinra and Celty's apartment.

Daily Life Arc
Namie hires Mairu and Kururi to spy on her little brother Seiji Yagiri and his girlfriend Mika Harima on her behalf. They later give her a detailed report of Seiji and Mika's most recent date and are paid money for their efforts, though they leave shortly thereafter due to Namie's clear ire at the events they described.

During martial arts practice at Rakuei Gym the following day, the twins befriend Akane Awakusu—a young girl who has newly joined the class—and decide to shoot the breeze together after practice ends. They encounter Shizuo, Tom Tanaka, and Vorona while walking down the pavement; 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 handle the next debt collection visit themselves, leaving the women behind to their conversation.

As Vorona is questioning the twins on the nature of their relationship with Shizuo, several lowlifes accost them with the intention of using them as hostages against the debt collector—that is, Tom. Mairu, Kururi, and Vorona easily drive the would-be kidnappers off by the time Shizuo and Tom return; shortly after the dust settles, Mairu and Kururi head home.

Adabashi Arc
With Taro Tanaka and Kanra absent from the Chat Room, Mairu and Kururi take the initiative to invite new members and liven up the online space. These members include: Kid (Mizuki Akabayashi), Pure Water 100% (Aoba Kuronuma), Chrome (Izaya Orihara), Sharo (Eijirou Sharaku) and Saki (Saki Mikajima).

Dragon Zombie Arc
Upon spotting Izaya outside Rakeui Gym, Mairu tells her brother to go die. Izaya sarcastically says that his feelings are hurt—and then avoids being physically hurt when he narrowly dodges an attack from Eijirou, the twins' martial arts instructor. Mairu and Kururi cheer on Eijirou's continued efforts to land a blow on Izaya until Eijirou's sister and co-instructor Mikage Sharaku intervenes, allowing Izaya to escape the scene. Eijirou suspiciously asks if Mikage still has feelings for Izaya despite what Izaya 'did to her' in high school; Mairu and Kururi eagerly await the answer, but Mikage's answer ultimately consists of a relentless hours-long attack against her brother for asking such a question in the first place.

Some time later, several members of Heaven's Slave tail Mairu while she is on her way to meet with Kururi. Eijirou thwarts their attempt to kidnap her; realizing that Kururi may have also been targeted, Mairu calls her sister and is told that Celty protected Kururi on Izaya's orders. After the twins reunite, they find Izaya on the streets and thank him for protecting them. They even declare that they will go so far as to "cry a little" when Shizuo finally kills Izaya—though they still insist they will laugh after they cry. The siblings all walk home together.

Durarara!! SHx2
Mairu and Kururi join Aoba, Akane, and two Raira freshmen—Yahiro Mizuchi and Kuon Kotonami—on a hunt for the Headless Rider, the hunt's purpose being to prove whether or not Celty is truly responsible for the abrupt spike in human disappearances. When the twins bring Yahiro and Kuon to Rakuei Gym, Mikage notices scars adorning Yahiro's hands and posits to Mairu that Yahiio must have earned those scars by smashing people's teeth. Mairu and Mikage both wonder how many teeth Yahiro would have had to break to earn scars like those.

Trivia

 * The name Mairu means "dance" (舞 mai) and "flow" (流 ru).
 * Mairu's surname Orihara means "fold" (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 share the same voice actress.
 * In the second season, they each have a different English voice actress.
 * Mairu is a competent martial artist who typically prefers to directly attack via punches and kicks, whereas Kururi often defends herself with items such as pepper spray and stun guns.
 * The twins' birthday is on Valentine's Day.
 * The Orihara siblings' maternal grandparents are deceased.
 * The twins have a brief cameo in the background of the first season's eighth episode, timestamp approximately 9:14.