Yuu Nanami, a 160-centimeter-tall first-year high school student with teal hair and eyes, joins the Earth Sciences Club during Mira Konohata's second year. Her commitment to meteorology stems from witnessing her aunt's distress after a flood damaged their home three years prior, though no injuries occurred. She views disaster prediction and mitigation as science's practical duty, not mere passion or hobby.

Initially serious and critical, Yuu dismisses the club's astronomy and geology activities as purposeless compared to meteorology's societal impact. She advocates bluntly for community-oriented projects, challenging the club's relaxed atmosphere and questioning their responsibility. This pragmatic directness contrasts with her occasional clumsiness and pronounced sweet tooth, which punctuate her otherwise reserved demeanor.

Her skepticism shifts through interactions with seniors Mira Konohata and Ao Manaka. After recognizing their genuine dedication to discovering an asteroid, Yuu fosters greater club cooperation, integrates into the group while prioritizing meteorology, and eventually becomes the third treasurer. She shares a close rapport with fellow first-year Chikage Sakurai, who teases her quirks. Yuu has two younger brothers with minimal narrative involvement. Her surname Nanami (七海) merges "seven" and "sea," while her given name Yuu (悠) signifies "permanence" or "leisure."