Codename wa Sailor V

Like Sailor Moon, Minako Aino is a normal 13-year-old schoolgirl until a fateful day when a white cat introduces himself to her and tells her she has the power to transform into the hero, Sailor V. Using a magic pen to transform, Sailor V fights the evil agents of the Dark Agency as she strives to protect the earth.

(Source: Kodansha USA)

Note: In 2004, the series was re-released in two shinsouban editions. The last chapter of the original print was split in two, hence it having 16 chapters. The chapter count follows the tankoubon release.