Gus was brewing a batch of love potion, when he accidentally tripped and fell into the cauldron. The boss entered his shop as he regained consciousness, and he instantly fell in love with her. Although the boss finds him to be clingy and annoying, he is a useful minion with credit cards. An unrequited love, Gus is trapped under the potion's spell.
Each mastery level adds another card to the cardpool for defeating Gus. Each of the bronze, silver, and gold mastery levels unlocks a new card, while it takes all three diamond and onyx levels to unlock a single card.