Manami Mamiya

Manami Mamiya is a supporting character in Durarara.

She appears early on as one of 2 girls that Izaya had tricked into committing suicide with and ends up getting humiliated when izaya learns of her petty reasons for wanting to end her life. She appears again, over a year later, as a member of Izaya's alliance in order to get an opportunity to get revenge on him for deceiving her.

Characteristics
Mamiya's appearance in the manga consists of a plain T-shirt and skirt with an light coat. She has shoulder length black hair, some of which is held back by a hair clip.

On her first appearance, Mamiya appeared to be very depressed, mentioning a recent breakup as her reason for committing suicide. After Izaya's deception and during her later appearances, Mamiya is shown to be very vindictive, vowing to kill Izaya over humiliating her. The novels describe her hatred for Izaya and desire for revenge being the only things that keep her wanting to live.

In terms of abilities, Mamiya doesn't possess any 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 Mamiya'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 attempt suicide.

Dollars/ Mika Harima Arc
Manami appears along with Izaya Orihara 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 Mamiya expressing her belief in the afterlife. Izaya appears exasperated with their responses, calling them hypocritical for wanting to end their current lives and expecting a better one in return. The unnamed woman quickly picks up that Izaya has no intention of dying which he admits to. The two girls are naturally offended but Izaya once again calls them hypocrites, saying they shouldn't care what anyone does to them since they were going to commit suicide anyway.

Izaya soon gets a call from an unknown individual, informing 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 to no avail. As Mamiya passes out, she swears vengeance on Izaya. The information broker expresses some joy at this as he tells her that hate alone can motivate her to live. Eventually, Izaya tells Celty to drop them off on a park bench somewhere.

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

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

Volume 11
Mamiya 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, Mamiya goes to her apartment to attempt to pursuade her to hand it over. Haruna agrees to hand over Celty's Head as it would allow her to continue her search for Takeshi Naujima.

Trivia

 * Mamiya's role in the anime was replaced by Rio Kamichika