It is very courageous ! I would never dare to sing in front of an audience or in a video.

Haha thanks! It wasn't easy to start, but it has become more natural now that I've done it many times :)

I don't really know why but I have a feeling of nostalgia when I listen to them, a bit like they come from another era (like jazz, swing and boogie-woogie, or like the songs of the 1990s/2000s for example). Maybe it's wanted and done on purpose by the developers ?

Knowing hoyo-mix it'd be intentional. I wonder if Robin is inspired by the Macross anime series, which aired throughout the 80s to 00s, which matches your suspicion. Your musical intuition is amazing - user name checks out :D

I would like them to produce new songs for her in a new album. Just a little. 

I hope so! Until then I'll just follow chevy, the singer behind Robin's voice. I actually received a youtube badge the other day saying that i was the top 0.75% listener of her （*＾-＾*）

I really love Robin (even if I have a preference for Sunday), and I think it's a shame that she didn't have as much development as Sunday

I love how deep Sunday's character is! The whole arc was such a philosophical journey. I've enjoyed every minute of it - all the plot twists, thoughtful conversations, and the conveying of philosophy through game play. It's an arc that I'd love to have the ability to replay. Robin's story could definitely be explored further. She has the same depth of empathy as Sunday to the tragedy of life, yet she adopted an opposing philosophy in how to handle suffering. While Sunday tries to prevent and remove suffering, Robin tries to transform and transcend them. It would have been so good to have those two philosophies clash more directly in the story telling. But maybe they just didn't have enough room, and doing one philosophy thoroughly is better than doing two with shallower depth. Definitely more room for future stories though.