Slime Boy is very shy. He was bullied through school and doesn't fit into most of Marada. He hides in the City Sewers, scared to go above ground. Even here he stands out! He rewards pets that are not shy. The higher your pet's Charisma is, the greater he will reward them.
NOTE - you are rewarded only once per pet. Trading for a pet with high Charisma stats list will NOT give you the prizes if they have already been rewarded.