Mafia

"The Mafia" is a west coast criminal organization allying with Mello which increases it's influence and intends to possess the Death Note and thereby gain power. How to Read 13 states that the organization is "being used by Mello."

Rod Ross
Rod Ross (Rodd Los in the Japanese version), real name Dwhite Gordon (Dwhite Godon in the Japanese version) is the head of the mafia. Ross follows Mello's orders. His name is easily known, so Light kills him.

Masaki Aizawa voiced Ross in the anime. Brian Dobson plays Rod Ross in the English Version. His name is also mentioned in L: Change the WorLd as the leader of an arms syndicate who deals with the mafia.

When asked by an interviewer who the most evil character in Death Note is, Ohba replied Ross, citing that Ross is "pretty darn evil" as he is the leader of the mafia.

Jack Neylon
Jack Neylon, whose real is name Kal Snydar (Kal Snyder in the Japanese version), is a member of Mello's crew. He is the holder and user of Sidoh's Death Note after it is given to them by the Kira Task Force. Before Mello joined, Jack was involved with drug trafficking and illegal weapons sales; neither of which charge stuck due to lack of evidence, despite four separate arrests. After Sidoh tracks down his Death Note, Jack performs the trade for the Shinigami Eyes. Light uses the Death Note to have Jack mail him the address of Mello's hideout. In the anime, the eye trade and first raid attempt were cut. He is voiced by Takashi Matsuyama in the Japanese anime and Ted Cole in English.

Other Mafia members

 * The mafia uses Zakk Irius (he uses his real name) to converse with the Japanese police at the airport. The organization kills Irius after he guides Soichiro to the correct aircraft. He dies by heart attack when the airplane lands; his crew wrote his name into the Death Note.
 * Glen Humphreys (Gurren Hangfreeze in the Japanese version), real name Ralph Bay (Langford Bid in the Japanese version), is a veteran gang member. The Death Note kills him prior to the police raid. The bonus volume How to Read 13 gives the following description: "An underling of Ross, an old timer in the Mafia group. He was killed with the Death Note during the sweep by the Japanese Police."
 * Rashual Bid, real name Al Meem (Yale Mison in the Japanese version), is a veteran gang member. When Kira eliminates the gang, he dies with most of the members. The bonus volume How to Read 13 gives the following description: "Another underling of Ross. Together with Humphreys, they were the main force behind the Mafia. He was killed when Kira massacred the Mafia group."
 * Miller is a member who was used as a test subject as to whether the Death Note was real during the exchange. He was killed because he was stealing drugs from the gang to sell, and was described as an "incompetent guy".
 * Jose is not affected by the Death Note which kills the other members, as only his alias was known. He fakes being dead and fatally shoots Soichiro, then later dies when the rest of the Japanese Police enter the room and shoot him.
 * Roy and Skyer are not affected by the Death Note, which kills the other members. They attempt to retrieve the Death Note, but the task force disarms and subdues them both, yet they later (presumably) die when Mello detonates the explosives which destroy the entire building.

In the Rewrite 2 special, the mafia are omitted, with Light's meetings with Kiyomi Takada and Teru Mikami moved to earlier on, and them instead being the ones to kill the SPK.

(Note that there are several other unnamed members of the Mafia in the series)