Shin Megami Tensei - Demikids has your starter Mon. The very first monster you get, and your partner for the entire game, only gains levels by absorbing other monster's exp through fusion. Taking into account that you'll likely need most of your other monsters for fusion, you'll usually end up just mass fusing dozens of cheap monsters to get your starter to level up. To put it differently: This is like getting a Pikachu that has to devour its opponent every time it wins a battle. By the end of the game, your starter is essentially a walking mass of hundreds of souls.