In the show, Naito's character is Touma Kamiyama/Kamen Rider Saber, who is a novelist, while Kawazu's character is Mei Sudo, Touma's editor and supports him and his various endeavors. Mei has no romantic feelings for Touma in the series. Mei is often shipped with Touma's ally, Rintaro Shindo/Kamen Rider Blades (played by Takaya Yamaguchi) and it is hinted at the end of the series that they started dating.
[Additional info via Crunchyroll News]