Prelude: Codename wa Sailor V
Kaitou Ace, or whatever it suits you to call him, is the leading male in the manga Codename wa Sailor V, a three-volume series by mangaka Takeuchi Naoko. Takeuchi is also creator of the more popular series Bishoujo Senshi Sailormoon, and CNSV is a prequel that flows directly into the larger series's storyline. The two are interlinked, and some of BSSM's characters--notably Usagi, her guardian senshi, and the Shitennou--have cameo roles in CNSV.
It tells the backstory of Aino Minako, an athletic, manga-reading, idol-chasing thirteen-year-old girl, whose world is turned upside down when she meets a white cat named Artemis. Artemis explains that Minako is really the sailor-suited fighter for love and justice, Sailor V, and that it is her mission to fight on behalf of the moon against the Dark Agency. The plot follows a similar crisis-showdown-resolution formula to its sister series. We follow Minako from her initial awakening as Sailor V to her true awakening as Sailorvenus. The series leaves off where BSSM begins.
Kaitou Ace plays a unique role in this series, and he reveals a total of four different incarntations during his brief appearance. He is in charge of the Dark Agency and, therefore, Sailor V's arch-nemesis. He also dons a mask and aids Sailor V in battle, becoming her savior. While Minako falls in love a good number of times throughout the series, Ace acts as her primary love interest. He provides a source of conflict and helps propel Minako towards accepting her true destiny.
CNSV was Takeuchi's second major project, following The Cherry Project. It was intended to be a short story, but recieved unexpected, massive popularity. This prompted Takeuchi to expand the story into BSSM. Takeuchi started this project in the summer of 1991, and it ran simultaneously with BSSM until November 1997 in Run-Run Magazine.1
1 Codename: Sailor V. Wikipedia.