Mairu Orihara (折原 舞流, Orihara Mairu) is one of Izaya's younger sisters and the twin sister of Kururi. Like her brother, she 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.
Mairu has brown eyes and long brown hair often tied back in a braid. 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 usually wears a similar outfit with an opposite color scheme.
Mairu is extremely energetic and outgoing and 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 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 her sister, Mairu regularly attends martial arts classes and is a skilled fighter. She uses her fighting skills more frequently than her sister.
For the most part, the twins and their brother 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 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.
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 thumb tacks 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 stopped the bullies from picking on Kururi, 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.
After seeing Celty 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, they place Egor in Simon and Denis's care. They call Shinra 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 notices them and chases the Toramaru members off. Kyouhei agrees to drive them to a safer place and calls 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, Anri, 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 her sister were hired by Namie to spy on Mika and Seiji 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, Tom, 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.
With Taro Tanaka and Kanra gone from the chatroom, Mairu and Kururi decide to try and invite new members to join which include Gaki (Akabayashi), Pure water 100% (Aoba), Chrome (Izaya), Shoro (Eijirou) and Saki (Saki).
Dragon Zombie Arc
Izaya passes by Mairu and Kururi'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 the twin's martial arts instructor, Eijirou Sharaku. 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 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.
Mairu and Kururi, along with Aoba, Akane and two freshmen: Yahiro and Kuon, 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.
- Main article: Kururi Orihara
Mairu and Kururi have been shown to have a very close and incestuous love for one another although Mairu appears to be the most outspoken about it. The two of them are rarely seen apart from each other and always make decisions on everything together from fashion to personality traits. The two of them even decided to have opposite IRL and Online personalities. While Mairu is usually the most talkative of the two in real life, online she is usually very quiet, saying little more than a word or two during conversations. However, these few additions to the chat room conversations that she makes are usually some form of innuendo.
- Main article: Izaya Orihara
Due to their parents usually being overseas for work, Izaya ended up practically raising the twins himself and says he feels guilty because he believes they turned out abnormally because of his influence. He claims that he never knew how to deal with them - a trait that they seem to share only with Shizuo Heiwajima - and that he finds them unlovable, even as humans, because of their oddness. He claims that if even he can't love them, then no one possibly could.
The twin's relationship with their older sibling is a fairly stable one. Surprisingly, they are among the few people who can actually annoy him, as he said he never knew how to deal with them while they were growing up. Despite Izaya's mild irritation with them, they seem to get along well whenever they talk however the twins have openly stated that they wouldn't mind Izaya getting injured or killed in some way (especially if it would somehow get them to meet their idol: Yuuhei Hanejima) which would imply that they are indifferent towards Izaya. However, at the end of Volume 09, Mairu admits that if Izaya does get killed she and Kururi would still feel sad before laughing.
- Main article: Aoba Kuronuma
While the twins and Aoba get along fine, Mairu's specific relationship with him is a bit of an odd topic. While Mairu does kiss Aoba immediately after Kururi does, it's very clear that the only reason she is interested in him is that Kururi is interested as well. Aoba, while perplexed, doesn't appear to mind this and seems to accept it with very little protest. It can be speculated that she and Kururi might like Aoba due to his similar personality to Izaya, the brother who raised them, and a need to earn his approval.
- She 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, she 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 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.
- Her and Kururi's birthday is on Valentine's Day
- The twins' and Izaya's maternal grandparents are deceased.
- The twins appeared briefly on the background in the 8th episode.