Lil' Monster

Lil' Monster, known as Gem Gem Monster in Japan, is a video game for the Game Boy Color handheld, published by Agetec. The game entailed players to train small monsters used for training and battling, while also collecting gems to help you, as the main character, in your quest to battle your brother who has become infamous for his evil deeds all across the land. Source: Wikipedia, "Lil' Monster", available under the CC-BY-SA License.