Raul is a bomb maker who is "the best of the best."
Raul is a bomb maker used by many criminals including Elias and "The Voice". He also works as the custodian at Hope Elementary School and is described as "careful, cunning, dangerous" by Elias as well as "the best of the best" at making bombs.
When enlisted by Finch to track down "the Voice", Elias led him to Raul who supplied "the Voice" with his ammunition and bombs. Raul admits he thought Elias was dead and refuses to supply him and Finch with information on "the Voice" when Elias explains about Raul's connection to "the Voice" and states that anything Elias does to make him talk "the Voice" will do worse to him if he does. Elias draws a gun on Raul who is unthreatened as they are surrounded by explosives. However, Elias has loaded his gun with hollow-point bullets so they will only harm Raul and demands to know how many bombs Raul built for "the Voice" and where they're located. Under threat, Raul gives up the locations which helps Finch realize that they are all within the jurisdiction of the 8th Precinct and are intended to draw out all of the cops inside the precinct. After Finch traces the detonation signal for the warehouse bomb, he and Elias leave Raul to resume his work with Elias and Raul exchanging a nod before they leave.
Later, Elias uses a bomb he got from Raul to kill "the Voice." (“Sotto Voce”)