As part of his induction into Khaderbhai's group, Lin starts spending time with Abdullah—a tough, young Iranian who has a lot in common with Lin. They're both angry, both fighters, and both crazy about Khader. It turns out that, like so many of the events in the novel, their friendship was planned and engineered by Khaderbhai; they do seem to genuinely like each other, though.
Abdullah is like a foil to Lin. (Forgot what that is? No worries, we've got you covered.) Even on a physical level he sets off Lin's characteristics.
Khaderbhai says that they look like brothers, which seems funny based on how different they are: "My hair was blond, and his was in black. My eyes were grey, and his were brown. He was Persian, and I was Australian. At first glance, we couldn't be more dissimilar" (2.9.151). The physical differences, though, hide the real similarities that the men share: both are angry, motivated by revenge, and living in exile under the wing of Khaderbhai.
At one point in the novel, Abdullah is killed in an angry mob, supposedly because the police believe he is Sapna. This is just at the exact time that Prabaker dies. However, while Prabaker—the good friend—really does die, Abdullah—the bad mamma-jamma—comes out on the other side safe and sound.
He shows up one day to surprise Lin, who has long believed him to be dead and gone: "It was Abdullah, my Abdullah, my dead friend, killed in a police ambush too many suffering months before" (5.39.233). What is the point of this weird disappearance? Abdullah's reappearing act shows the power of Khaderbhai to manipulate everyone around him, even after he himself is dead (or is he?).
Abdullah might be Lin's foil physically, but he's also an important contrast to one of Lin's deepest principles: respect for human life. They both might be violent, but the fact that Abdullah has killed is an important distinction for the characters:
Abdullah and I were very much alike. We were men of violence, when violence was required, and we weren't afraid to break the law. We were both outlaws. We were both alone in the world. And Abdullah, like me, was ready to die for any reason that seemed good enough on the day. But I'd never killed anyone. In that, we were different men. (2.10.155)
The characteristics and actions that define both Abdullah and Lin are not very flattering, but the one big difference is about restraint and respect. Even though Abdullah is a fairly likeable guy, he doesn't have the self-control and all-around courtesy to not kill. At least, that's how Lin interprets it.