Wataru Takagi is a newcomer to the Tokyo Police Criminal Investigations Division 1, having transferred after the bomber case about three years ago but by the time Shinichi Kudo first established his reputation as a detective, about two years ago. His mentor was a senior agent called Date, who died one year before the present timeline. In his last moments Date entrusted Takagi his notebook, which had a ring for his girlfriend Natalie Kuruma hidden. He kept Date's notebook as a personal motivation to be just as good as Date. It was first a year later he realized that Date's girlfriend was Natalie, but it was too late to give her the ring, because she committed suicide soon after Date's death. He was first introduced in an Anime original case at an on location film set.

Ever since his joining the force, Takagi _(like so many of his other colleagues)_ has harbored romantic feelings for Miwako Sato. While Sato appeared to have taken a liking to him as well, Takagi was easily discouraged by the way she dealt with other male colleagues, including their superior Inspector Megure _(though in that case it was more a daughterly affection to a father figure, since Sato's father had died when she was very young)._

In time, thanks to several severe incidents - and despite the interference of the other officers in the department, chiefly Ninzaburo Shiratori -, their professional relationship grew more personal, and finally Sato and Takagi shared their first kiss after he had been hospitalized in the course of a difficult case and kissed the second _(and third!)_ time in Episode 683 when he almost died in a bomb accident.

(Source: Detective Conan World)