Aki Hiyami operates as the Ninomiya Squad's Operator within Border defense. Her pale complexion, light platinum blonde hair styled in a chin-length bob with bangs, and rounded blue eyes distinguish her appearance. She wears the standard Operator uniform in her Trion Body; her civilian attire typically layers a light-colored long-sleeved shirt under a black blazer, paired with high-waisted wide-legged trousers or a skirt and black boots.

Initially introverted and nervous, Hiyami maintained distance from others until Mirai Hatohara's guidance helped her develop warmer social ease. Residual guardedness initially strained interactions with teammate Shinnosuke Tsuji, though they cultivated a close rapport. She exhibits fierce protectiveness toward Tsuji, cautioning others against "messing with our Tsuji." During combat, her level-headed reputation endures through dispassionate, concise communication. Observant in private, she challenges teammates' decisions when needed and patiently guides tactical choices during missions.

Hiyami engages in squad social activities like birthday celebrations and casual outings with Masataka Ninomiya, Sumiharu Inukai, and Tsuji. She addresses Inukai with "-sempai" and Tsuji with "-kun," while both use her nickname. Despite Hatohara's defection, Hiyami maintains respectful ties, referring to her as "Hatohara-sempai" without visible resentment. Outside her squad, she harbors a crush on Kyōsuke Karasuma, around whom nervousness disrupts her usual composure.

As an Operator, she analyzes combat scenarios to intervene tactically and steer decisions, blending observational acuity with silent guidance techniques that reinforce tactical learning.

Her personal preferences include sweet and sour pork, yogurt, and baths. A distinctive trivia note designates her hair as the "slickest" among female characters due to added screentone effects in illustrations, referenced as "slick" in her character profile.