How's this for an introduction?:
He was admired and feared by the rich. He was respected and mythologized by the poor. His discourses on theology and ethics, held in the courtyard of the Nabila Mosque in Dongri, were famous throughout the city, and drew many scholars and students from every faith. No less famous were his friendships with artists, businessmen, and politicians. He was also one of the lords of Bombay's mafia. (2.9.91)
Abdel Khader Khan, also known by the honorable nickname of Khaderbhai, basically functions as a father figure to Lin, with all the messy complications the role implies.
Khaderbhai enters Lin's life pretty mysteriously, and almost immediately charges him with the task of teaching his young nephew English. It's kind of a strange thing to do, sending a kid you care about to live with a foreign stranger in a slum, but that's the way Khaderbhai rolls.
And the move is a smart one. By showing Lin that he trusts him with great responsibility, he gives Lin the feeling that he can trust Khaderbhai. The two gradually get closer, with Khaderbhai inviting Lin to his philosophical discussions at his home and helping him get medicines for his makeshift clinic in the slum.
Unfortunately for Lin, he's not the only one that Khaderbhai plays daddy for. The father figure has got some serious sibling rivalry bubbling up around him because he surrounds himself with renegades who've lost their homes and families, ripe for a leader to fill their long lost dads' shoes.
Lin is jealous when Khader likes other people, of course: "I felt a tiny twitch of jealousy. The sense of well-being bestowed by Khaderbhai's benignant smile was as intoxicating as the heady mixture we'd smoked in the hookah pipe. The urge to be a son to Abdel Khader Khan, to earn the blessing of his praise, was overwhelming" (2.14.85). Khaderbhai has the ability to bring out a competitive spirit in his followers, making them all want to be the best so they will be worthy of his praise.
Eventually, Lin has to grow up, leaving his idealized image of Khaderbhai behind when he realizes he's been playing him all along. It's when they're in the mountains of Afghanistan, just before Khader's death, and the huge difference between what Lin thought Khader was and what he really was is just too much for him. It's unforgiveable.
Lin refuses to join Khader on his final mission, revealing Khaderbhai's commitment to his own mission above all else, even above his relationships: "I was still angry that I'd put so much of a son's love into Khader [...]. I was angry that he'd considered me expendable, to be used as a means to achieve his ends" (4.34.141). All of Lin's rage is directed at Khaderbhai's questionable ethics, in which the ends justify the means.
Khader's personal philosophy is that you can do bad things, like killing or lying, for good reasons. And that somehow turns them into good actions—or at least forgivable ones. The problem is that, for Lin at least, they're not forgivable. Khader is willing to lose friendships (they're "expendable") in order to achieve his goals.
For all of his fancy cars, houses, and lifestyle—not to mention his smooth talking and deep philosophical discussions—in the end, Khaderbhai is a crime boss. He's a thug, a gangster, a thief, and a murderer. He's just dressed up in fancy outfits while he does it, which is enough to blind those around him, for a while at least.