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I've read that the more you progress on a guy's relationship, your phone wallpaper will change? I'm "Lovers" with Hikaru but the phone wallpaper is still Haato. Bug?

No, the wallpaper system is scrapped from final release. 

I see. Would've been a cute detail but oh well.

Indeed it would be cute! Maybe cool features like that could be added in the late future? Since they're already working on so many cool stuff, like artbook and voice. :)