Drinks all of the coffee: La Muerte is pure evil without her caffeine fix in the morning. Need I say more?
Brings up adopting a pet: La Muerte likes the idea of it, but she knows that she would want something like a Persian or Siamese cat but Xibalba would want a pitbull.
Kills the bugs: Xibalba, because La Muerte likes seeing how stupid he looks when he does it. He gets all ninja and junk, spraying the bug spray everywhere and screaming random Spanish swears.
Cooks the meals: La Muerte, due to the fact she’s concerned for her husband. If it were up to Xibalba, he would probably live off of nothing but churros and wine.
Starts getting into holidays way before they should: Surprisingly, Xibalba. Especially the gift-giving ones, because he loves spoiling his wifey :P.
Initiates the couple selfies: La Muerte. She has her ways to get her stubborn husband to pose in them.
Forgets the birthdays and anniversaries: Xibalba, obviously. He always ends up a day late (which ends up with him sleeping on the couch but his wifey eventually joins him) or a day early (which causes La Muerte to get all sappy on him).
Always ends up with too much junk food after grocery shopping: Xibalba has a sweet tooth BECAUSE REASONS.
Nicknames the other: La Muerte started it, so Xibalba got revenge.