This is an amazing vampire hunt adventure game, with multiplayer capabilities. The original game pak had a special solar sensor which allowed to have more power while playing in the sun. Obviously this feature won't be available in an emulator, but you will enjoy this great game anyway, nicely packed in five languages.
If you liked the game, and reside in US, you can buy the solar-powered gamepak from Amazon, at this page.
Download link for Boktai - The Sun is in your Hand (Multilanguage)
Size: 8 MB
Platform: any GameBoy Advance emulator