From AnimeNewsNetwork:

She calls herself Sexy Voice. To most of the world, she's just Hayashi Niko, an ordinary schoolgirl - but when time permits, she lives a double life as a hired investigator/spy. To help her, she has recruited the rather unlikely assistant she calls Robo, a geeky lump of a man who doesn't really know why he feels the need to do everything she tells her to. He just does it. Together, the odd team solve a number of cases for Sexy Voice's employer, an aging gangster.