Nomi Yagami is the wife of Yōji Yagami and mother of Yūji Yagami. Her exceptionally youthful appearance frequently causes others to mistake her for decades younger, sparking comedic conflict as her son Yūji develops an inappropriate attraction and his homeroom teacher, Akira Yokkaichi, becomes infatuated. Nomi remains largely oblivious to these romantic feelings.

Characterized by naivety, Nomi dotes excessively on Yūji and displays deep affection for her husband through public romantic gestures that embarrass her son. Her devotion manifests in domestic routines, such as preparing special Valentine's Day chocolate cakes for Yūji and offering daily morning kisses. She becomes emotionally vulnerable when her maternal role is questioned, reacting with distress when Yūji doubts their biological relationship due to missing childhood photos; she later produces photographic proof of his infancy, reaffirming their bond with a heartfelt embrace and affirmation of unconditional love.

Nomi's unwitting actions often create awkward situations. During a thunderstorm, comforting Yūji in his room inadvertently triggers his internal conflict. Influenced by television, she inappropriately questions Yūji's friends about his virginity. When misled by Yokkaichi into believing her marital affection stresses Yūji, she suppresses her natural happiness with Yōji and offers Yūji compensatory kisses for routine activities.

She maintains a traditional household presence, welcoming Yūji's friends for meals and participating in school events like parent-teacher conferences. Her interactions with others, such as Yōji's subordinate Mitsuko Nanase (who explicitly compares her own attractiveness to Nomi's), remain limited. Nomi's core traits—youthful appearance, marital devotion, maternal dedication, and unintentional comedic influence—remain consistent across all adaptations, with no significant character development.