It's in a fantasy western style setting where vampires are problematic (and referred to as Immortals) and we play as the optimistic very good boy, Django
What made the Boktai games so unique was that the cartridges themselves had a little solar panel that played a big role in the gameplay and made it important for you to go outside and get some sun
Which yeah I know, going outside and all that and I can't give anyone shit for that because I emulated the games with mods to control how much sun it'd receive
and tbh I've found that GBA music in general works best when it's a mix of synth waves and samples that don't try too hard. Boktai was pretty dang good about that.