Sera and Serena (ムー＆スー, Mu & Su) are twin sisters and the default children of Rosalind and Ray. They live upstairs in the clinic. They are also total opposites, One of them likes going outdoors, while the other likes staying inside and reading. Even though they are two different people, they act as if they are one, in terms of gift-giving, so only one gift is needed. In the second generation, they are bachlorettes for Aaron but have trouble understanding what a wedding is.
Serena: Thanks! I love this!
Sera: Thanks! I love it!
- Tomato Juice, Pudding, Omelet Rice,
Sera: You're giving us this?
Serena: You are?
- Trash, Tomato, Pumpkin Pudding, Dried Sardines, Sardine, Chocolate Cookie
Serena: Sorry, I don't like this very much.
Sera: I hate it, actually.
- Sardine Sashimi