Gal*Gun 2

In order to unlock Yuko Yureino you must get the phone number from every girl in the free missions and keep doing free missions for a while afterwards, eventually you should see a mission called "A spooky SOS from a ghost~ Oooh~" beat it to obtain Yuko Yureino's phone number and unlock her.

Give treats to a girl until you have a full heart. Once you have a heart you can invite her to your room. When she's in your room you press ( - ) on the controller and the oldschool dating sim will start. It's also showing your rank on the right.