Earthworm

 Mimizu (ミミズ Mimizu, "Earthworm") is the alias used by the former leader of a group called "Amphisbaena" which ran an illegal gambling parlor. She is one of two major antagonists featured in Volume 09 and is one of two recent additions to Izaya's alliance.

Characteristics
A rather well endowed woman in her late 20s, Mimizu has mid-length blonde hair and a slender build. She has been seen wearing revealing clothing that emphasizes her breasts.

Although she tries to keep a calm and collected demeanor, when the right buttons are pushed, Mimizu reveals a much more manic mentality due to her obsession over a former classmate of hers. When backed into a corner, she panics easily and constantly pleads with a non-existent "owner" to help her. She is shown to be somewhat capable in a fight but doesn't hold a candle to some of the more experienced characters in Durarara!!.

Background
In the past, "Amphisbaena" was a group that was founded by Izaya and another student named Nakura during their high school years. Like its present version, the group also ran an illegal gambling ring but on a much smaller scale than what Mimizu uses it for (the players placed bets on baseball games much like how people placed bets on horse races). Izaya and Nakura used false identities while they managed Amphisbaena and Mimizu was one of the people they used to represent them. After Izaya and Nakura cut off all ties to Amphisbaena, Mimizu took control of the group herself and eventually turned it into a full blown gambling parlor. She seemed to have a strange obsession with Nakura and believed herself to be his lover. When Mimizu took over Amphisbaena, she made it seem like Nakura (who used the alias "Lizard" when he and Izaya were still in charge of the organization) was still in control, most likely to use him as a sort of scapegoat if things went sour. She refers to Nakura as "Owner" frequently which further cements her strange obsession with him.

Dragon Zombie Arc
Amphisbaena had grown to a larger organization (mostly made up of women) than it had previously been. Eventually, their actions began to encroach on the Awakusu's territory which prompted immediate retaliation. The Awakusu executive Shiki sends Izaya Orihara to gather information on Amphisbaena as a last ditch attempt to avoid a full on gang war.

Mimizu takes note of Izaya's actions and sends her subordinates to corner and capture him. Izaya is then bound to a chair and has a potato sack tied around his head. Mimizu tries to get Izaya to reveal what he knows but gets only silence. She brings up the prospect of torturing Izaya's sisters to get him to talk but she is soon interrupted by another unexpected guest.

A man named Shijima who is the leader of another group called "Heaven's Slave" along with several other thugs barge into the room with the intent of killing Izaya where he sat. Mimizu asks how he found out about their locations and Shijima explains that his men managed to steal information about her from Izaya's laptop. As Mimizu and Shijima argue over what to do with Izaya, Mimizu gets a sudden call from her cellphone. When she answers, the voice on the other end introduces himself as the real Izaya Orihara.

Mimizu is initially dismissive of his claims until the "Izaya" on the phone begins to imply that the man they have in the chair isn't just the wrong man but is actually Mimizu's "Owner." Unable to contain her fear and her curiosity, Mimizu loosens the ties on the potato sack around Izaya's head when even more people barge into the room.

Mimizu and Shijima are then faced by Mikage Sharaku, Ran Izumii, and several members of the gang Dragon Zombie. Ran reveals himself to be the Izaya on the phone while the man in the chair somehow manages to shake off the ropes that bind him and pulls off the potato sack to reveal...Izaya himself. Izaya explains that he had allowed himself to get kidnapped and had purposefully leaked some of his information in order to lure the two groups he was investigating into a corner.

Mimizu tells her subordinates to kill Izaya and his allies but they refuse to respond. Mimizu takes a closer look and sees that their eyes were glowing red which reveals that they were attacked and placed under the control of Haruna's Saika.

Out of desperation, Mimizu attacks Izaya but is easily incapacitated by Izumii. After Shijima and his men have been dealt with, Izaya orders Haruna to use Saika on Mimizu to force her to join him. Although it isn't explicitly shown, it is assumed that Mimizu along with Amphisbaena is now under the control of Haruna, and by extension, Izaya.

Durarara!! SH
Mimizu makes an appearance at the very end of the second Durarara!! SH light novel, alongside Shijima.