Zero Woman: Abunai yûgi (1998)

In this installment of the Zero Woman series, Rei, the number-one assassin of the Tokyo Police Department's secret Zero Division, has been assigned to take out a notorious gangster responsible for dozens of murders. At the same time, she must protect the gangster's mistress from the hitmen who want to keep her from telling the police what she knows. At first, both women are upset about the arrangement, but they eventually come to appreciate each other.