William Jackson Harper, best known as the hyper-intellectual Chidi on the gone-but-not-forgotten sitcom The Good Place, projects an amiability and decency that is impossible to fake. In Eboni Booth’s affecting new play Primary Trust, which opened Thursday at Roundabout Theatre Company’s Off Broadway Laura Pels Theatre, he manages to command our attention despite the recessiveness of his character. Kenneth is a middle-aged Black man living in a mostly white town east of Rochester, New York, and he addresses the audience in the halting manner of an individual who’s most at home in the shadows and out of the spotlight.

Orphaned as a 10-year-old boy, Kenneth spends his days working in a used book store and his nights sipping mai tais in the neighborhood tiki bar — with only the company of an imaginary friend (Eric Berryman), in whom he has built up a full biography. Kenneth is shaken out of his lonely routine when the bookstore owner (Jay O. Sanders) sells the place and moves away, and a waitress at the tiki bar (April Matthis) suggests he apply for an opening at a local bank like the one that had employed his long-dead mother — a decision that is fraught with challenges for a man so wracked by past trauma that he’s withdrawn from even the most basic social interactions. (The staff at Wally’s tiki bar has even taken bets on his job, with CIA agent among the leading contenders.)

Booth has crafted a deft portrait of a trauma survivor at a critical moment of inflection, just when he might be tempted to come out of his shell, just a little. It’s a deeply humanist approach that operates on a defiantly human scale, and she makes no effort to morph Primary Trust into a capital-I Issue play interrogating the societal issues that have led Kenneth to his particular predicament. The only nod comes in Marsha Ginsberg’s striking set design, where giant models of small-town buildings (including abandoned Main Street storefronts) dwarf the cast.

Hers is a stylized approach, with repeated breaking of the fourth wall and an onstage musician (Luke Wygodny) ringing a front-desk-like bell to mark quick changes of scene and shifts in time, Richard Nelson-style. While these meta devices are effective, some elements of her play strain credulity. Matthis is a hoot playing more than a dozen different characters, from bank customers to an ever-changing variety of tiki-bar servers, each with a highly individualized accent — but the astronomic turnover rate at that local bar is as hard to swallow as a week-old pu-pu platter.

However, such shortcomings are well-disguised by the power of director Knud Adams’ production, which moves swiftly through its 95-minute running time to reach an ending that is far more hopeful than we might have dared to wish. Even before we learn about Kenneth’s full backstory, Harper has conveyed the character’s sweet guilelessness and fundamental decency. We cannot help but root for him.