Nanaki Moriyama attends Homurahara Academy Elementary School alongside classmates and friends Illyasviel von Einzbern, Miyu Edelfelt, Suzuka Kurihara, Tatsuko Gakumazawa, and Mimi Katsura. She is the younger sister of high school student Nanami Moriyama, who attends the same academy as Rin Tohsaka and Luviagelita Edelfelt. This close bond with Nanami drives Nanaki's strong protective instincts, particularly towards her sister.

Generally laid-back and caring, Nanaki possesses a quick temper that flares when provoked, especially if Nanami is involved. She reacts strongly against anyone causing her sister distress, leading to her dislike of Rin and Luvia due to their tendency to inadvertently harm Nanami during magical activities, though she forgives readily with apologies or resolved conflicts.

Her family lineage carries a blessing of innate, generational physical capabilities that remain dormant under normal conditions. This potential surfaced when she mastered the Gakumazawa Style martial arts with exceptional ease, overpowering her adult instructor. This feat led Illya to recognize Nanaki as a combat prodigy, highlighting her latent prowess despite her young age and typical elementary school demeanor.

Physically, Nanaki has pink hair styled in pigtails and typically keeps her eyes closed. She actively participates in her friend group's everyday school life and occasionally observes the magical conflicts involving her peers, though she remains largely detached from the supernatural elements themselves.