Mika Harima

 Mika Harima' (張間 美香, Harima Mika'') is a friend of Anri Sonohara, who had been looking for her after she had disappeared from her home. She was a stalker, and had been stalking Seiji Yagiri until he accidentally "killed" her. Seiji then asked his sister to transplant the head of Celty onto Mika, however, Namie, Seiji's sister, merely gave her cosmetic surgery to look like Celty's head. She was presumed dead until episode 12 when she announces that Seiji hadn't really killed her, and that she is in love with him. Later, Seiji accepts her love simply because she looks like the head he so desparately fell in love with as a child.

In the manga, it is said that Mika is an intelligent girl from a rich family.

Characteristics
Originally, Mika had brown hair and eyes but after her surgery, her hair color was changed to a lighter brown with grey-green eyes along with much paler skin. Her usual street cloths consist of a white collared shirt, a pink skirt and tie, and a white hat with a heart on it.

Her normal personality can be described as hyperactive, cheerful and obsessive. As evidenced by her relationship with Seiji, Mika is very possessive of the person she loves and will go to great lengths to gain their affection, even to the point of invading their privacy. She was even willing to abandon school and even her own identity for him just to make him happy. While most would be quick to write Mika off as a stalker, Mikado states that that isn't entirely accurate. While Mika does exhibit stereotypical stalker tendencies, all of her actions have been taken out of a genuine love for Seiji as opposed to most stalkers who only act out of selfish desires. Mikao is also shown to be very caring towards her friends as she was still worried for Anri during the Yellow scarves incident.

Mika is shown to be very skilled with tools such as when she was able to break into Seiji's apartment and how she was able to set up several wiretaps and cameras in several people's apartments. She is also regarded as the best cook among most of the female cast.

Background
Mika was Anri's best friend during middle school and the two were often seen together. One day, they were being accosted by a group of thugs but were saved by Seiji Yagiri. This caused Mika to develop an instant crush on Seiji and she began stalking him. Mika eventually breaks into Seiji's apartment while he was out and finds Celty's Head. When Seiji returns ,he attempts to kill Mika by slamming her head against the wall but manages to survive. When she wakes up, she is greeted by Namie Yagiri, Seiji's older sister, who explains the situation between Seiji and his love for the head. She proposes that Mika would get plastic surgery to look like the head and to take memory altering drugs so that Seiji would love her. Mika agrees to the surgery and learns from Shinra that the head's name is Celty and that she should use that name when talking to Seiji. While Mika completes the surgery, she refuses to take the drugs and manages to escape.

Dollars / Mika Harima Arc
She bumps into Mikado the day he comes to Ikebukuro on the street with a mysterious scar around her neck, before running. While walking around Ikebukuro, Mika was also mistaken for her old self from behind by both Anri and Mikado, but both backed off after seeing her new face and believing her to be someone else. She was eventually found by Seiji Yagiri as the girl with a scar on her neck, and Seiji came to call her "Celty". She and Seiji are shown spending time in various areas of the city however, Mika knows his sister is constantly keeping watch over him. One day, she tries to get Seiji to run away from his sister's surveillance but the Headless Rider sees her and chases after her to get a better look. In the process, she bumps into Mikado again and asks him to save her in a panic. Mikado runs with her and brings her to his house and offers to bring her to the police but she refuses. The next day she is left in Mikado's apartment alone while he goes to school but gets a call from Izaya Orihara while she was waiting. He explains that she is going to be discovered soon so she runs into the street only to almost get run over by Saburo Togusa, but he stops before he hits her.

Later that night, Kyohei Kadota, at Mikado's request, brings Mika to meet Celty. When Celty Asks what Mika's name is, Mika introduces herself as Celty as well. Celty soon realizes that Mika doesn't have her head after all and screams in frustration. After the Dollars meeting, Seiji attacks Celty with a taser, making her fall to the ground, and retrieves Mika from Togusa's va. As the two walk away, Seiji encounters Mikado he thanks him for reuniting them. However, when Mikado brings up Mika's "death" Seiji pulls out a knife and starts trying to attack Mikado in a rage. Celty gets up to defend Mikado and attacks Seiji but Mika interfers by jumping in the way and finally revealing she is Mika Harima. She then explains everything that happened to her after Seiji attempted to kill her, including the plastic surgery from Shinra. As Seiji falls down in disbelief, Mikado remarks that, in a way, Seiji and Mika seem to genuinely care for each other and that they show their love in similar ways. Mika and Seiji return to school the next day with Seiji professing that he will accept Mika's love until he finds the real head.

Saika Arc and Yellow Scarves arc
Mika plays minor roles during both of these arcs, only appearing to offer cautionary advice to Anri about Takashi Nasujima. She also gets worried for Anri during the Yellow Scarves incedent and asks Mikado about it. However, Mikado doesn't know anything.

Hollywood Arc
Mika and Seiji attend the Hotpot party held by Shinra and Celty. She assists Celty, Emilia, Anri , and Erika with the cooking and reminds everyone to thank Seiji since her cooking skills only exist for him.

Daily life arc
Namie attempts to undo the plastic surgery done to make Mika look like Celty's head. Realizing what this would do to her and Seiji, she threatens Namie with the extensive amount of evidence she managed to gather about her and Izaya's activities. Mika states that she obtained all this information by bugging the house and the phones thoroughly, although Izaya noticed it easily.

Namie asks why she found out so much about not just her and Izaya but about Mikado, Shizuo, Anri, and everyone else and why she never did anything to stop it if she knew what Izaya was planning. Mika states that while she never did anything to help Anri or her other friends, she did what she could to keep Seiji out of harm's way. She explains that, despite his harsh nature, Seiji does feel guilty about almost stabbing Mikado during the Dollars meeting and would even try to help him if he found out about what was going on. Mika made sure to keep Seiji oblivious to what was happening so that Seiji wouldn't put himself in danger.

Trivia

 * While in the anime, Mika leaves Mikado's apartment after a phone call from Izaya and ran into (more like ran over by) Kadota and his gang, in the Light Novels and manga Kadota and his gang, having already beat up and interrogated by an employee of Yagiri Pharmaceuticals, notice a message on the perp's phone commanding to get Mika Harima dead or alive along with the Mikado's address and simply beat everyone to her.
 * Namie never went to Izaya to ask him to find Seiji (and Mika, by extension) in the Light Novels and manga.
 * Celty initially encountered Mika by chance as opposed to being set up by Izaya Orihara in the Light Novels and manga.


 * Mika has had Izaya Orihara's office tapped.
 * Volume 4 of the light novels rates the cooking skills of several female characters in Durarara with Mika scoring a perfect 100.
 * Namie Yagiri kissed Mika Harima to get an indirect kiss from her brother, Seiji. This probably means that Mika and Seiji kissed and Namie was already aware.