As with applications of this kind, My Boy! Free - GBA Emulator doesn't include any games, only the possibility of playing them if you can find and save the ROM to your device, which is relatively easy to do. You'll have to go to specific websites to download them, but there are many sites online where you can find Gameboy Advance ROMs.
The emulator is perfectly compatible with most of the games, including Castlevania (some of the best titles in the saga are Gameboy Advance games), Super Mario games, and even many of the Pokemon series. All of them work perfectly on this emulator. On the settings menu, you can adjust different aspects of the app, like sound, graphics, and even the control system.
My Boy! Free - GBA Emulator, even as a free version, includes every feature of its paid counterpart. The only difference is that in the free version you can't save the game at any point in time (which you can do with the paid version), and instead, you have to do it from inside the game. Still, this is a superb emulator.