^
Does it have to be Japanese for a reason? Because that's not really a style that's always embraced there as much as it is in Korean music. You're also more likely to come across voices that aren't as high compared to Japanese music, and a less quirky sound.
But um, if you don't mind Korean, you want Casker.
I'll make a playlist for them with the songs I like actually so you can get an idea, although some will be more upbeat. In random order.
Here
I think you might also like Yozoh. Are you at jps, because there's actually a circle of korean artists that are this style, and jps has a lot of material.