I agree. Friends can come in all forms. The team could develop some mini-games for the protagonist and Adrian to play together, leveraging the existing "Find Adrian to play games" quest. This would be another way to become friends.

After arriving in Japan, share the girls there with Adrian, and after the Japan trip gets 3 stars, the protagonist can take other girls to travel to Japan.

The devs already answered the question about Japan unlock a long while ago, but for short, they will rework the storyline of some characters and Adrian is one of them. For now they are apparently reworking Nixxara. As for the Japan trip they also plan to make sure that player can unlock both by sharing AND not sharing. But for now there's only 2 ways to unlock it... one you share and the other one is by modifying the save file of the game. 

PS: you don't need ANY special program to modify the save file, your pc own Notepad app is more than enough, all you need to do is understand how it works the save file. I prefer to point it out so you don't risk on trying to download some sketchy apps from anywhere  and put yourself to the risk of getting a malware. Trust me it's not very difficult to understand the save file, and if you're afraid of losing your save you can always make a back up copy and try your hand at it. Worst is could happen is the game not booting up and if this happens, just delete the save file you killed and put your back up in place with the right name