Thank you for asking @persephones-valentine I'll take the opportunity to talk about a bit about Saya's band Kirye Eleison and their members.
So, Saya met them at uni, she was already into goth and metal music but they were the one that motivated her to make a band. They get along pretty nicely with each other, but Saya is the most distant of them, she doesn't stay to celebrate after the gigs and when they hang out she's mostly quiet letting them talk and such. The girls really care for her and they know Saya is really peculiar in the way she share her time with others, so they let her be trusting she will come with new songs.
Viper: Main guitarist. She’s the most extrovert of the members, with a strong personality and a lack of patience to deal with annoying fans (she’s always ready for a fight). Her family owns a bar, and she’s the older of 5 siblings. She had to help raise her siblings, work in the bar, and keep up with her studies. She had always dreamed of becoming famous with her band, and as soon as she learned about Saya’s talent, she proposed to herself to make her famous.
Ivy: Bass. The closest to Saya. She had a band at high school until they disbanded for unknown reasons. Her pacifist behavior works as a nice contrast with Viper’s aggressive attitude, but she’s still very mischievous and likes to play pranks on Viper and Saya. She’s a bass prodigy but prefers people to ignore it.
Vultur: Drums. Viper’s best friend. Due to the lack of a manager, they work together to organize events and places for their band to play. She’s overly honest, but her tone sounds extremely sarcastic, which confuses people. She is the most experienced of the group at surviving supernatural encounters since she’s a fan of the occult, and people fear her ability to cast a curse. She lives at a Temple due to her family tradition. Some rumors say she killed someone.
And for the question about the Kaiba bros. Yes, Mokuba finds out really quickly, practically on the second week Saya started working at KC. He uncovers her like in metal since this girl loves listening to music so loudly. He doesn't show up to the gigs until Saya gives him permission, Mokuba knows she need to trust him first and Saya is alway running away from everything so it takes him some time to finally see her sing.
For Seto, that's another story. It takes him months to finally know about Saya’s band. And I don't blame him, Saya is extremely secretive around him. So when he finds out it's the most shocking experience for him.
I hope to be able to draw that moment for Seto soon, because it's one of the most important moment for both of them. It's so exciting to even think about aaaaaa.
Thank you for asking again! And if anyone has more questions please let me know ^^ I'll be more than happy to answer (and I hope to have a drawing to reply to it.)