Who is Amar in the novel, Allegiant?

Amar was Four's former instructor at Dauntless. He spotted Four before he was named, Four, and took him under his wing, helping him learn to fit in and leave his Abnegation ways behind him. Four credits Amar for saving his life on more than one occasion over the years. It is a shock for Four to learn that Amar is still alive, as Amar had reportedly been killed in an explosion when he'd jumped off of a Pire and was killed below. On reflection, Four realizes that they never did recover much of a body after that. Amar has been living and working for the Bureau ever since that time. He tells Four that he had to leave because they'd just come out with a serum that could detect the Divergent...and he was one.