Fourteen-year-old Nico spends her free time on a cell phone-based dating service, but not for the reason you'd think. Hoping to someday become a fortune teller or maybe even a spy, the girl uses the dating service to determine what people are like merely by listening to the sound of their voice, and soon, she manages to accidentally befriend a Yakuza boss. Now Nico has the opportunity to use her curiosity, intellect, and keen sense of hearing to foil the schemes of kidnappers, crooked athletes, and forgetful assassins - but is it a good idea for a junior high school student to work for the Yakuza?
This is a bit of a gut-feeling rec, but the main characters from Sexy Voice and Forget-Me-Not remind me of each other. Both are untraditional young female detectives, with excellent reasoning skills. Neither are run-of-the-mill episodic detective manga, so if you enjoyed one, you might like the other as well.