Scene: Woman
This I think is an optional sidequest but it gives an insight to Rikiya and a childhood friend.
When Rikiya gets to Komurocho he is lost and Kazuma has to find him, once found he will want to go sightseeing. So take him sightseeing and eventually he will want to go to Millienum Tower but you can't go in. Rikiya will apologize and want to here about Kashiwagi in the Park. After that Rikiya mentions wanting to go to a bikini bar but Kazuma isn't in the mood. So have Kazuma return to Serena. A scene will play out and then you can go hang out with Rikiya more. Choose to do more sightseeing with Rikiya.
Take Rikiya to Asia on Pink St. Pay to get in and watch the scene. Once you leave Rikiya notices the dancer outside and goes up to talk to her. He mistakes her for his childhood chrush Natsumi but it's Akiho her unknown little sister that Rikiya never heard of. Walk around a little bit and Rikiya will get a call from Akiho, go to where she is and save her. The repeat the process, Rikiya will get a call and go save her. This happens three of four times. Kazuma starts to get annoyed. And will follow Akiho and over hear bashing Rikiya with her friends. Kazuma then steps in, talks to Akiho and takes her to where Rikiya is so she can see how far he will go for her. (I like to think Kazuma dragged her by the hair.)
When Kazuma and Akiho get to where Rikiya is, he is on the floor being beat up begging them to let the debt Akiho has go. Kazuma tells Akiho that Rikiya WILL die for her and Akiho tries to step in. Rikiya says it's too dangerous and Kazuma backs him up in the fight against the loan sharks. Once that fight is over Kazuma is smoking Rikiya talks to Akiho. She reviels that she is Natsumi and Rikiya tells her he has always known. He tells her to come back to Okinawa with him and she can start all over. She agrees. <3