Ultraviolet: Code 044 Season 1 Episode 2

...First Love

044 is ordered to infiltrate the assembly of Phages in the Neo Tokyo sewer system to find their leader, King. There she finds Luka, a young man who has made a promise to kill her. They fight, however, before 044 can kill him, he chooses his own fate by diving into the sewer. In this encounter something deep within 044 is awakened and thus she rescues Luka, whom she was to kill.