Nagisa first appears working in the Marine House seafood restaurant as a waitress, catching Ryuji's attention with her t-back bikini. However, although she seems sweet and innocent, she actually suffers from Dissociative Identity Disorder, and has an evil side who calls herself Yasha.
She is a member of the Midnight Angels gang under Junya Akutsu, and is his girlfriend. Her mental disorder arose after he raped her in the past. Ryuji saves her life, and she eventually overcomes her problems and becomes his girlfriend. They are still together in GTO.
Briefly seen in the manga, Nagisa Nagase is Ryuji Danma's attractive, grad-school girlfriend, where they first met in the GTO prequel, Shonan Junai Gumi. In the manga, when Eikichi Onizuka is exceptionally angry, and does not wish to teach the class, he is shown sitting behind his desk contemplating, while the students watch a lesson given by Nagisa through a videotape, albeit reluctantly, through its footage.
In the anime she first appears when Onizuka and Danma are having a conversation in a bar after Onizuka's interview. Onizuka tries to dive right into her bosom when Danma grabs him right by the collar and shortly starts throwing punches at Onizuka for being a bit of a lech towards Nagisa. She does not appear in the live-action.
In the Shonan Junai Gumi OVA (episode 4), Nagisa is seen with long, straight dark hair and goes to the prestigious Fujisawa Girls School. A hanya tattoo appears on Nagisa's back, near her left shoulder, whenever her alternate personality Yasha surfaces and she rides a black Kawazaki Eliminator motorcycle.