Ui Nakatsugawa is a cheerful and energetic second-year high school student living in the rural town of Yamabiko. She has long orange or peachy-pink hair, often styled with a heart-shaped hairpin and a small side ponytail on her right, complemented by a yellow hair ribbon. Her eyes are bluish-green, and she stands about 156 cm tall. Ui typically wears her school uniform: a pale peachy-pink shirt, brown tie, red skirt, and knee-length brown socks.

Known for her outgoing, friendly, and sometimes naive nature, Ui is widely adored. She possesses an extraordinary appetite, frequently consuming large quantities of food like cheese-flavored Umaibo snacks and showing a particular fondness for cold, sweet treats such as parfaits. While generally optimistic, she occasionally experiences feelings of selfishness, especially when desiring exclusive attention.

As a child, Ui was a crybaby heavily reliant on her older brother, Hajime Nakatsugawa. She struggled to accept his marriage, perceiving it as him being stolen away, but later reconciled by offering belated congratulations upon learning of his impending fatherhood.

Ui's relationship with Kazuma Hoshino began when she helped him after he got lost in the mountains; during this encounter, Kazuma slipped and accidentally kissed her. This moment became a recurring memory for Ui, and she gradually developed romantic feelings for him. They eventually mutually confessed their feelings and entered a romantic relationship.

Ui maintains close friendships, notably with her protective childhood best friend, Ibuki Hinata. Other significant relationships include the shy shrine maiden Madoka Kōmoto, the Tōdō sisters (Tsumugi, Koyori, and Kasane), and reserved senior student Hina Sakai. Peers generally address her familiarly using nicknames like "Ui-chan" or "Ui-san".