Serena miscalculated how much money she had left in her account and ended up betting more than she had at the Blackjack's casino. Unfortunately for her, he isn't the forgiving type, and seems to have decided to have her pay him back in another way. He's not sure what that way will be yet, so he'll just store her inside his body—indefinitely.

Blackjack is capable of storing his victims inside himself via absorption, and can keep them alive for as long as he wishes.