Mairu Orihara

Mairu Orihara (折原 舞流, Orihara Mairu) is one of Izaya's younger sisters. She has a twin named Kururi. Like her brother, she's 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. This causes a stir in the class and she is bullied briefly until she responds to one of the bullies by threatening her with thumbtacks as well as taking her to the bathroom and stealing her bra. Later, the sisters are assumed to have framed the bullies for having drugs by sneaking them into their backpacks.

She kisses Aoba after Kururi does, claiming she was happy to get an indirect kiss from her sister. Along with her sister, she is known to be a big fan of Shizuo's brother Kasuka, stating that she would gladly see Izaya die in front of a truck for a chance to meet Kasuka.

Anime
There is a cameo of both of the twins walking side by side behind Simon and Walker. There is another cameo when Mikado checks the 'lost and found' notebook at the ending of episode 8 when both of the twins wrote down Kasuka, then drew chibis of themselves in the corner of the page.

Both of the twins appear in of Episode 25, where they search for Kasuka Heiwajima (Yuhei Hanejima), but Shizuo Heiwajima wanted his brother to do his job so he kept them away from him.

Light novel
Volume 04: 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 who did this to her desk. 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 tacs from the back of the classroom and makes her way over to the bullies. She grabs one of them at random and slams her down on the floor, threatening to pour the tacs down her throat if she didn't fess up who destroyed her desk. The girl points to her friend that is trying to leave the classroom. Mairu throws the thumb tacs with amazing accuracy at the door handle which causes the bully to freeze up. Mairu then grabs her shoulder and drags her outside and into the girl's bathroom expressing a sudden desire to "get to know her." The girl isn't 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. She warns Aoba that Kururi is actually more aggressive than she is despite how she acts. After seeing Celty in action against the cops on the news, Mairu and Kururi decide to try and meet the black rider and go on an adventure through out 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 Igor who politely asks the two of them for directions to a place called Russia Sushi. After getting the directions from Shizuo, they place Igor in Simon and Dennis'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 meeting 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 Dennis the remaining 800,000 yen to pay for him to hire Celty to transport an unconcious Igor for a day. (In short, they effectively returned the 1 million yen Celty lost). Mairu and Kururi also join the chatroom apparently having recieved the link from Namie. Unlike her real life personality which is outgoing and talkative, online she is very quiet and rarely says more than a few words. However, her tendency to mention obscene topics is still present as she is occasionally censored by the chatroom's content filter. Her screen name is "Mai."

The next night, the two of them somehow got into a confrontation with a few toromaru members when they were asking about the headless rider. Kyohei notices them and chases the toromaru members off. Kyohei agrees to drive them to a safer place and calls Togusa to pick them up. As soon as they get in the van, the Toromaru 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 Toromaru members with her shadow, Mairu, Kururi are dropped off near a police station but head back to the overpass with Aoba to see what was happening. They watch in awe as Ruri and Igor beat up the Toromaru members. Later, they attend the hotpot party held at Shinra and Celty's aparment.

Volume 08 : 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 (who do you think?).

Volume 09 : 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 in from there.

Voice Actresses

 * Japanese: Eri Kitamura
 * English: Candalee Whitmore