Kenta can't be affected by Karin giving him blood. He isn't miserable anymore thanks to their love.
Turns out Elda found the wrong ceremony. It actually makes boobs bigger. Thankfully the ceremony was interrupted. Karin already has large ones and them getting bigger wouldn't be doing her any favors.
Karin's blood increases faster if accumulative misfortune befalls people near her.
To get Victor to leave them alone, Karin suggests all the Markers bite him. Without self-deception (Carera drains lies), ego (Henry drains pride), exasperation (Ren drains stress), self-hatred (Anju drains envy) and misery (Karin injects happiness), Victor's personality does a 180 and he becomes happier. Due to how long these emotions fostered in Victor, his blood tastes gross to them.
At the very end, a silent montage shows Karin and Kenta having a happy married life.