He is the brains of the operation to Vandemar's brawn, though not exactly lacking in the latter department himself. He enjoys eating priceless Asian ceramics.
In "Neil Gaiman's Neverwhere", Croup and Vandemar are hired by the angel Islington to murder all but one of a noble family of "openers" from London Below, and bring the sole survivor back so that she may create a door back to heaven. In the end, the two are sucked through a door to who-knows-where. It's a safe bet they survived, though.