Manami Mamiya

Manami Mamiya (間宮愛美, Mamiya Manami) is a supporting character who appears early on as one of two girls whom Izaya meets for an online suicide pact. She appears again, over a year later, as a member of Izaya's group in order to find an opportunity to get revenge on him for deceiving her.

Appearance
Manami's appearance in the manga consists of a plain T-shirt and skirt with a light coat. She has shoulder length black hair, some of which is held back by a hair clip. Her appearance in the anime is vastly different, presumably to make her stand out more. Her hair is a short reddish-brown with one side tied into a small pig tail. Her outfit consists of a black sweater with a pink jacket that appears to have the sleeves ripped off. She also wears glasses and sports a necklace that looks like an eye.

Personality
In her first appearance, Manami appeared depressed, mentioning a recent breakup as her reason for wanting to commit suicide. Shortly after meeting Izaya at a Karaoke bar for the arranged suicide pact, Manami learns that Izaya never actually intended to end his own life and was only tricking them. After Izaya's deception and during her later appearances, Manami is shown to be vindictive, vowing to kill Izaya for humiliating her. The novels describe her hatred for Izaya and desire for revenge; as being the only things that keep her wanting to live.

In terms of abilities, Manami possesses no abnormal strength or skills. However, she is shown to be very clever at times, being able to convince Haruna Niekawa to hand over Celty's head in order to expose it to the public.

Background
Not much is known about Manami's past before the start of the series other than that her boyfriend dumped her for another woman. This caused her to sink into a deep depression which drove her to want to attempt suicide.

Dollars/ Mika Harima Arc
Manami appears along with Izaya and an unnamed woman in a karaoke booth sharing one last drink before they end their lives. They each discuss their motives for committing suicide as well as the best method to use. Izaya then asks what the two of them will do once they die, with Manami stating she believes that death is just an expanse of nothingness, just a piece of darkness, however, that will be much better than this. Izaya seems exasperated with their responses. The unnamed woman quickly picks up that Izaya has no intention of dying, which he admits to. The two girls are naturally put-off, but Izaya once again calls them hypocrites while saying they should not care what anyone does to them since they are going to commit suicide anyway.

Izaya soon receives a call from an unknown individual who informs him that they would arrive soon. Izaya hangs up and points out several key details about the room they were in, primarily the person sized suitcases underneath the table as well as the fact that he had drugged their drinks earlier. The two girls assume that Izaya is planning to sell them to human traffickers and cry out for help, but the drugs in their drinks kick in. As Manami passes out, she swears vengeance on Izaya. Izaya expresses some joy at this as he tells her that hate alone can motivate her to live. Later on, Izaya tells Celty to take the girls and drop them off on a park bench somewhere.

Daily Life
When Izaya's hospitalization is announced on the news, Manami uses this as her opportunity to get revenge. When she visits him at the hospital, Izaya is initially unaware of who she is, saying she was one of many girls he played the suicide prank on. Upon hearing this, Manami simply attacks him with Izaya easily disarming her despite his injuries. As Izaya slowly remembers who she is, he starts laughing uncontrollably, impressed that Manami managed to hold on to her hatred for so long. Reinvigorated with his love for humanity, Izaya offers Manami a job for her to bear witness to what he was about to do. Manami accepts, seeing it as a way for her to get another opportunity at revenge.

Adabashi Arc
Manami is made Namie's assistant after Izaya cements his alliance. Izaya explains that even Namie needs help processing all the paperwork that has built up over the past few months.

Saving Izaya/Haruna Returns Arc
Manami decides to steal Celty's head to try and throw a wrench into Izaya's plans. As it was Haruna's turn to guard the head, Manami goes to her apartment to attempt to persuade her to hand it over. Haruna agrees to hand over Celty's head as it would allow her to continue her search for Takashi Nasujima. Once she is given the head, she hurls it into the street to be spotted by people.

Final Curtain Arc
Manami copies Izaya's hard drive onto a USB and discovers the location of one of 'Yodogiri's' apartments. She goes to deliver the drive free of charge for the sake of ruining Izaya's life. However, when she gets there, she spots an injured Shinra and questions him on his relationship with Izaya. Satisfied with his response, she lends him a shoulder to help him walk and bring him to Celty. On the way she asks him any weaknesses or anything he hates, in which Shinra provides he is surprisingly frail in heart and detests pain. She witnesses the fight Izaya and Shizuo has, and while Shizuo is stunned she helps Kine carry Izaya away and joins Kine in bringing him to the hospital in order to see his end or to ridicule the fact he is saved by a monster if he should live.

A Standing Ovation with Izaya Orihara
It is mentioned by Haruto that he has met her at some point, when they 'went to Tokyo', but it is currently uncertain when, how or the details of that exchange. However, it is certain that Manami still keeps in contact with Izaya in some form for yet unknown reasons.

Trivia

 * While the first season of the anime depicted Rio Kamichika as the girl Izaya meets for an online suicide pact, Manami and another unnamed girl are the original victims in the light novels and manga. Later on, in Durarara!! x2, Manami is introduced into the anime as well, along with her past regarding Izaya due to the critical role she plays in the future plot.
 * Ryohgo Narita implied in a tweet from 2015 that Manami may be somehow connected to Masashi Mamiya, a vampire from the Naritaverse series Vamp! who shares Manami's family name.