For starters, we're not really sure who Brint is. At the beginning of the book we think he's a psychiatrist, but by the end we understand that he works for the government. But is he a good guy or a bad guy? It's not clear, and that's kind of the point.
Adam notes that he's not "entirely comfortable" with Brint, even though "he seemed sympathetic and friendly" (2.7). This is kind of how we see Brint, too. We can't really figure out exactly what it is we don't like about him, but there's just something fishy. We never get to know him as a person, despite the fact that, until the very end, he's the only character other than Adam who speaks in the present moment.
Most of what we know about Brint (other than the transcript at the end) comes from Adam's point of view, which means we get a very one-sided view of him. This is true of all the characters in the book, but especially so for Brint because Adam is experiencing him in the present moment. For example, Adam thinks, "The eyes stared at him occasionally as if the doctor – if that's what he was – were looking down the barrel of a gun, taking aim at him" (6.2). Think about how this sentence might have read if told from Brint's point of view: perhaps, "I watched him intently, hoping my focus on him would show him that I meant no harm." Just a thought. The objective (meaning it doesn't come filtered through Adam's perspective) transcript at the end does let us come to some of our own conclusions about Brint; for example, that's he's a bit of a meanie.
Brint sure asks a lot of questions. It's his job, of course, but the way he asks them is so harsh, to someone who is clearly in pain. He speaks only in very short sentences throughout. As readers we can pick up on the fact that Brint is almost just a plot-mover. We don't know him very well, we're not attached to him, so what purpose does he serve? He asks Adam the right questions so that we can find out the answers.
One last thought. This might be a stretch, but Shmoop wonders if Brint and Mr. Grey might be the same person. Some evidence makes it seem unlikely: why would he be writing a recommendation for his own job status; and wouldn't Adam have recognized him? But other signs make us wonder. Brint is labeled "T" in the transcript, and Mr. Grey also went by the name Thompson. Mr. Grey and Brint both use the word "terminate" to describe someone's death. What other pieces of evidence can you find to support one side or the other?