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.

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. The next day she wasn't at the opening ceremony for Raira. Mika was also mistakened 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". One day, she tries making Seiji run away from his sister's surveillance but the Headless Rider sees her and chases after her to get a better look.

Mika bumps into Mikado again but this time screaming "Save me!". Mikado runs with her and brings her to his house. Mikado offers to bring her to the police but she refuses. The next day she is left in Mikado's apartment alone. Then she gets a call from Izaya Orihara 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 brings Mika to Celty and Mika says her name was "Celty", shocking the real Celty. After the Dollars meeting, Seiji attacks Celty with a taser, making her fall to the ground, opened Togusa's van and retrieved Mika. When Seiji encounters Mikado he thanks him but immediately points his blade at him when he brought up Mika's "death" and eventually starts stabbing at him. When Celty gets up and sees Seiji attacking Mikado, she slashes at Seiji but Mika interfers by jumping in the way and finally revealing she is Mika Harima. She then explains that she was stalking Seiji and one day went to his apartment and saw Celty's head in his possession, Seiji didn't want her to see it so he "killed" her. Luckily she was still alive and Shinra Kishitani gave her plastic surgery to look like the head and had told her that her name was now Celty. Seiji falls down in disbelief but Mikado remarks that Seiji really did care for Mika in a way and that the couple resembled each other.

The next day, Mika comes back to school with Seiji.

Saika Arc
She warns Anri about Takashi Nasujima harassing girls.

Yellow Turban Arc
She asks Mikado where Anri was but Mikado doesn't know.

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. In the color illustrations, it ranks the girls cooking skills with Mika scoring a perfect 100.

Izaya in the Hospital / Mika vs. Namie
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.

Differences between LN, Manga, and Anime

 * 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.

Voice Actresses

 * Japanese: Mariya Ise
 * English: Carrie Savage Lee

Trivia

 * Mika has had Izaya Orihara's office tapped, it is unknown if it is still tapped.
 * In the Light Novels, she seems to have some large mysterious source of information and connections that rivals Izaya.