There's no artist in particular :3 I've purchased a number of background and sprite packs to use with creating VNs, and then I just edit the colours and stuff myself so that they look more unique to my games since the packs are available for anyone to buy. I just don't like using assets as they come (especially sprites) if I can help it. So with this, I changed all the colours on the sprite I picked for Castor/Castoria. I think the original artist selling the sprite packs is Japanese, same goes for the seller of the background packs. The UI is mainly just made by me though in a mixture of GIMP and ComiPo :3
If I had the ability, I'd draw stuff myself, but I'm not very good at art >.<