Ai Saeki is a student sharing the protagonist's class and a member of the school Tennis Club. She possesses brown hair, brown eyes, stands 1.58 meters tall, weighs 47 kilograms, and has body measurements of 85-60-82.

Saeki nurtures romantic feelings for her classmate and fellow tennis club member, Shuuichi Sugiyama. Paralyzed by shyness, she finds directly expressing these feelings difficult. This inner conflict intensifies due to her awareness of Shuuichi's close bond with his stepsister, Mio Sugiyama. Witnessing an intimate encounter between Shuuichi and Mio in the locker room heightens Saeki's sense of rivalry and complicates her emotions.

During a summer tennis team training camp, Saeki's interactions with Shuuichi and Mio escalate. Observing their persistent closeness fuels her resolve not to lose Shuuichi to Mio. While her primary romantic focus remains on Shuuichi, she engages in sexual activities with Mio. These encounters occur both with and without Shuuichi's involvement, reflecting the multifaceted and evolving dynamic among the three.

Her surname "Saeki" (佐伯) combines the characters for "help, aid" (佐) and "count; eldest brother; chief official" (伯). Her given name "Ai" (藍) means "indigo". She appears in the sequel visual novel *Fault!! Service ~Aratanaru Rival~*, continuing her role in the narrative.