Jimmie Sasaki remains a mystery from the beginning of the book up until the end. When we meet him, he's a track fan attending college with Louie. He spends his time convincing people to send money to Japan to help the poor, but what he's really funding is Japan's war efforts against America with American money.
It turns out that he's a fake student, 21 Jump Street-style. He's almost forty and under FBI investigation. Ha.
When Jimmie resurfaces in Yokohama, he literally says to Louie, "We meet again" (4.19.16), officially making himself a character from a cheesy spy movie. Louie goes back and forth between believing that Sasaki is watching after him and believing that Sasaki doesn't give a flying maki roll about him. No one knows where this dude's loyalties lie, though at one point, "he began to sound like he was rooting for the Allies" (4.22.16). But really—no one's ever sure.
After the war, Jimmie is sentenced to six years in Sugamo Prison, where he tends his own vegetable garden. Whether he was an "artful spy" (5.35.15) or not remains unknown.