Select the "Extras" option at the main menu. Then select the "Cheats" and enter one of the following codes.

Right when you start playing story mode, only fight in the bars like 7th heaven and the limit where a lot of girls are. After the first fight where a guy is taking up for a girl and you win, the girl should message you and say something sexy. go to a bar a few more times and another girl will start flirting with you. When she does, your first girl will get mad and start yelling at her. Then you will fight the first girl as the second girl. whoever wins sends you another message. then the only way you ever talk to them again is if you get another girl.