Tonpa mentions Umori and his brothers as regulars of the Hunter Exam. Umori first appears during the First Phase of the 287th Hunter Exam, Satotz's marathon, where he and his brothers aid Tonpa in driving off some of the weaker contestants, such as Nicholas, showing that they've known Tonpa for some time, and have likely worked together with him before.
During the Fourth Phase of the exam, Imori stalks and attacks Killua in order to claim his badge. However, Killua easily takes his badge then overpowers and holds Amori hostage to force Imori and him to surrender their badges. Killua then throws two of the brothers' 3 badges into the woods since he doesn't need them, failing the trio.
Umori and his brothers partake in the 288th Hunter Exam, only to be easily knocked unconscious together with the rest of the candidates in the First Phase by Killua. Seeing the tremendous gap in abilities between Killua and themselves, despite the boy being much younger than them, they decide that they will never retake the Hunter Exam again.