Who is Jake T. Casto from Immortal in Death and what is their importance?

Casto is the Illegals investigator, a handsome and very charming man and a very dirty cop. Casto uses his position in Illegals to get close to the people who know how to traffic illegal drugs, and he finds out about Immortality. He decides that being a cop just does not cut it for him any more; he knows that within a few years Immortality will be restricted, but he intends to make his fortune from the very beginning.