Ether in Chapter C

A cat "Ether" who frequently visits Fisica's house. He is cute when asleep but a bit naughty when awake. Fisica sometimes left the window open for him to enter at any time.
— In-game description

Ether is a male cat that is presumed to be Fisica's because of the constant visits he makes to Fisica's house.

He also appears in the artwork for Just a Fairy Tale, and after completing three challenges in the song, the story would say:

Who knows where it came from, but there was a kitten who would often sneak into the lab. Fisica thought it was super cute. Even if it made trouble she didn't mind. She even gave it a weird name - Ether. And so it was, one person, one cat, often on the rooftop happily passing the time reading.
Just a fairy tale 3

Ether on the Just a Fairy Tale artwork