Eraldo Coil is the name of the second most famous detective in the world. In reality, it is an alias that is used by L in certain types of cases.
L also told the Japanese Task Force that he used the alias as a method to further conceal and hide his true identity. Aiber was allowed to pose as Coil to trick the Yotsuba Group during the Kira Investigation. However, in Death Note Another Note: The Los Angeles BB Murder Cases, it is revealed that Eraldo Coil was an actual person before giving up his name to L.
Despite the Yotsuba Group being unaware that Aiber was not actually Coil, it is said in Death Note: Another Note that it is common knowledge that L, Eraldo Coil and Denueve were one and the same.