Eraldo Coil

"… L engaged in a war with the real Eraldo Coil, and the real Danuve, and emerged victorious, claiming their detective codes."

- Mello

Eraldo Coil (エラルド・コイル, Erarudo Koiru) is the name of the second most famous detective in the world. The actual Coil is no longer active at the beginning of the series, and L uses their name as an alias for some of his cases.

Plot
The Yotsuba Group contacts Eraldo Coil to "uncover L's identity," and Watari informs L and the Task Force. Yotsuba "went through two agents to try to keep" themselves secret, but Watari determines that the request came from Masahiko Kida, Yotsuba Group's Tokyo office Vice President of Rights and Planning.

Soichiro Yagami describes Coil's reputation:
 * "Eraldo Coil is said to be one of the best detectives out there, besides L. He's famous for taking any job, as long as the money is good, and he's near the top in terms of locating people…"
 * "… And nobody knows what Coil looks like, either…"

L tells the Task Force that he is Coil. He uses the aliases Eraldo Coil and Deneuve, the second and third most famous detectives in the world. When people who try to uncover L's identity, they usually fail because they contact one of the other two.

Aiber poses as Coil to trick the Yotsuba Group. As Coil, he calls Kida, surprising him since Kida took steps to hide his identity. "Coil" asks for a much higher payment to take and complete the case, and a high payment for his silence even if they don't hire him. He tells Kida to think it over and call him back. Kida informs the other members of the Yotsuba group and they decide to hire him. That Coil has a reputation of being motivated by money is a contributing factor, since if he was the type to "act according to a stupid notion of justice instead of money, then we're all screwed," according to Namikawa. Ooi suggests offering even more money up front, which they do.

Once hired, "Coil" feeds the Yotsuba group select information. After three days, he delivers a report informing them of various things, such as that the NPA has withdrawn from the Kira investigation, and that Watari is L's representative. The report also gives some advice to help them supposedly avoid L's detection, such as suggesting that they spread out the deaths that benefit the Yotsuba group so they don't all happen on Fridays and Saturdays.

"Coil" sends a second report to Yotsuba, informing them that "Misa Amane may know L." Kida calls Coil, and Aiber (pretending to be Coil) agrees to join them face-to-face for additional payment. As Coil, Aiber sits with the Yotsuba members during Misa's interview and questions her about L suspecting her of being the second Kira. Their questions are rehearsed, and the members believe it.

In other media
In Death Note: Another Note: The Los Angeles BB Murder Cases, Mello reveals that Eraldo Coil was an actual person before giving up their name to L. Sometime after Aiber poses as Coil to the Yotsuba Group, it becomes "common knowledge" that L, Eraldo Coil, and Denueve are all actually the same person.

Trivia

 * Coil's gender is not specified in the series in Japanese. However, since Aiber poses as Coil, it's suggested that Coil is male.
 * In addition to using Coil and Danuve's names, L had "at least three digits' worth" of others.