Roth had deals with Claude Wome for diamonds imported from Africa, one time leading to being arrested.
Events of GTA IV
The Diamond Deal
Roth also had connections with Pegorino Family Capo Ray Boccino. He entered a deal with Ray, which took place in the The Libertonian. Isaac, Mori and two others stood face-to-face with the two men dealing for Ray, Johnny Klebitz and Niko Bellic. While Mori was looking at the diamonds, Luis Fernando Lopez shot Mori from a window overlooking the transaction. Isaac ran to Mori's side and tried to help him but when he realized he was dead, he snatched the diamonds and ran while Johnny took the money. Soon Isaac was caught and knocked out by Luis, who then stole the diamonds and took them to Gay Tony.
Isaac thought the ambush was set up by Ray and threatened to have him killed. In retaliation, Ray sent Niko to kill Isaac at his base in the Majestic Hotel.
Isaac Roth may be a reference to Hyman Roth, a character from the movie The Godfather Part II.
Isaac is one of only two characters to interact with the three protagonists (he does the diamond deal with Niko and Johnny, and he gets knocked out by Luis), the other being Roman Bellic.
Isaac has a minor case of rhotacism, a speech impediment where one has difficulty or inability to pronounce the letter R. However, when listening to his lines, the impediment is not consistent; he can fully pronounce the R in Mori Green's name, but not the Rs in "Of course, no problem." This is somewhat consistent to real-life speakers with rhotacism as they often try to correct the impediment by forcing themselves to speak with Rs rather than (in Isaac's case) Ws.