Reporters in The Idolm@ster 2 are very similar to reporters in real life. They want to follow around celebrities or other interesting people to get a scoop.
Any time that you complete an audition, live, or festival in an area, you have the chance of the reporter in the area (if there is one) deciding to follow you around. You can tell in advance whether there is a reporter in the area, and who it is, by looking in the lower left part of the Schedule Select screen next to the pencil icon. There will either be dashes (if no one) or the reporter's name in kanji.
Once you get a reporter, they will will follow your unit around for 4 weeks (including the week you first get them). The reporter magnifies the positive or negative effects you get for doing well (or poorly) at various aspects of the game. For instance, if your idol unit successfully passes auditions/lives/festivals while Ariake Jin is following them, the reporter will write a positive story about your unit which gets your unit more fans for that win. But if you fail an audition/live/festival while the reporter is following your unit, they will write a negative story about your unit and the effect will turn negative.
Below shows how each reporter affects an idol unit.