The Demon Slayer is the first game from Megami Tensei series that translated into English. Megami Tensei is a fairly popular series in Japan, but only this gameboy version has been translated. The Demon Slayer is a traditional RPG, but it has a feature which allows the monster to join in the party.
Player can combine two monsters together to create a new, stronger monster.
But this gameboy version also has a VS mode, which allows people to use their monsters to battle each other. In this game, player can negotiate the monsters during the battle to make them join the party. Weather the monster will joining the party or not is really depends on the answers of the questions player choose. And there's also a disadvantage of this game, it's the level problem. There is a big different in the level of monsters between one place to another, so player have to take quite awhile to level up the characters before they can head to the next place.