Tennōji Nagisa, known as Nagi, is a second-year high school student sporting brown hair usually in sporty pigtails and large circular glasses essential for her severe nearsightedness; daily tasks prove difficult without them. Removing her glasses and letting her hair down dramatically alters her appearance, revealing a more conventionally attractive look. A previous attempt to change hairstyles in middle school ended when excessive paternal attention became irritating, leading to a return to her signature style. She typically wears her school uniform under a light brown sweater and uniquely colored socks.

Nagi blends intelligence with sharp wit, often delivering quip remarks. Body consciousness fuels repeated diet attempts driven by weight concerns, though these frequently fail due to her dislike of exercise and frustration over friends like Tōru's petite frame and Yūko's proportions. She exhibits high sensitivity to temperature: cold weather prompts her to bundle up completely except for her eyes, creating a distinctive look, while heat also causes discomfort. A mischievous, almost sneaky side emerges when she enjoys startling the easily frightened Yūko or observing Tōru's teasing. Despite this scheming tendency, reliability and maturity define her role within the group, often serving as the "tsukkomi" counterpoint to her friends' comedic antics.

Her habits include biting straws after finishing drinks and placing her hands behind her head after meals. She maintains close friendships with classmates Run Momoki and Yūko Nishi, later extending to Run's childhood friend Tōru Ichii after a smoother initial acquaintance compared to Yūko's awkward introduction. This dynamic positions Nagi as both observer and participant, balancing Run's ditsiness, Yūko's timidity, and Tōru's protectiveness.

In the OVA *A Channel + smile*, established traits continue. "Mountain of Pancakes" depicts her involvement in a café visit where Miho works, encountering disruptions from Yutaka's arrival, alongside Tōru bringing her cat Tansan to school. "A Picture of a Wish" shows her assisting Yūko during illness with friends, followed by group participation in New Year shrine visits. These scenarios demonstrate her supportive role, dry humor, and interactions within the core friend group, maintaining consistent personality and relationships throughout all appearances.