Mr. Buttons is a chinchilla with genius level IQ after he was given memory enhancing serum. He decided to hide his intelligence so he could steal the serum and give it to all the other chinchillas in the lab. Until then, he will feign ignorance until he has everything he needs. If this means helping the boss with his lab experiments, then that's what he'll have to do.
Each mastery level adds another card to the cardpool for defeating Mr. Buttons. 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.