Antonapoulos is the ultimate Other. All of his character traits set him apart – he's an immigrant, he's deaf, and he's also a little (or a lot) crazy.
The one who always steered the way was an obese and dreamy Greek. In the summer he would come out wearing a yellow or green polo shirt stuffed sloppily into his trousers in front and hanging loose behind. [...] His face was round and oily, with half-closed eyelids and lips that curved in a gentle, stupid smile. (1.1.1)
But here's the thing: most of what we know about this "obese and dreamy Greek" comes from Singer. And, of course, Singer is a bit biased. In fact, Antonapoulos functions as Singer's Singer. That is, Singer imbues (gives) Antonapoulos the qualities he wishes to see in him, just like all the other characters do to Singer.
In the end, it seems like Singer makes the same sorts of assumptions about Antonapoulos as the other characters in the novel make about Singer himself – that he's wise and all-knowing and that he "understands" completely. In reality, Antonapoulos isn't all that and a bag of chips. Instead, he's a glutton, a kleptomaniac (or compulsive shoplifter), and a drinker. He's rude, he's crude, he treats those around him like dirt, he's lazy – oh, and he's full of it.
Singer doesn't see any of this, which makes us wonder: are we missing something? We never really know the inner Antonapoulos at all, so perhaps we should trust this guy who claims to be his BFF. But… probably not. Sure, he acknowledges some of Antonapoulos' bad points: for instance, he recalls with shame the time when Antonapoulos totally bailed on their deaf friend Carl. But over time, Singer begins to see only the good in Antonapoulos – he idealizes him past all recognition.
Those ugly memories wove through his thoughts during the first months like bad threads through a carpet. And then they were gone. All the times that they had been unhappy were forgotten. For as the year went on his thought of his friend spiraled deeper until he dwelt only with the Antonapoulos whom he alone could know. (2.7.16)
Thanks to these kinds of narrative details, we are able to see things Singer does not. And in fact, Singer's idolization of his friend reaches such heights that he transforms Antonapoulos into a sort of weird religious figure in a dream. So yeah, kind of creepy.
Singer and Antonapoulos are perhaps the two truest friends in the whole novel, and not even they can know each other completely. Maybe it's a comment on twentieth-century life – and this sense of alienation seeps into the entire novel. In the end, Singer does to Antonapoulos what everyone else does to him, and turns Antonapoulos into exactly what he wishes him to be. Will these people ever learn?