Sis, Sister, Big Sister, or whatever you call her is Shimeji's older sister of unknown name. Throughout the series she concludes various experiments, often involving unusual fishes.
She is always seen wearing a white lab coat. In some arts that are not part of the manga she is seen without her lab coat (see ./double pesce), it is unclear if it is canon. Her black long hair is usually in two ponytails (very cute). She also has a silver fish-shaped hair clip (very cute).
She has dropped out from the university so she can devote more time to research.
Her room is such a mess! Shiitake-chan couldn't even find her coat there.
Sis is knowledgeable in the way this world works, i.e. fish mechanics, fish math, dream world, etc. Here's a glimpse of her thoughts shown in ss06:
Antagonism theory — u/bouncepaw
I think Sis is really suspicious. The true goals of her experiments are never really explained. She might as well have evil (?) ideas. After her experiment in ss08 the world has become weird. Was it part of her plan? Pehaps.
The more tkmiz keeps people from knowing her name, the harder it is to give one. People's expectations are getting higher and the interest only gets bigger. Like, what if you give her a name but it'll be too weak? Or maybe tkmiz is a genius and everything is thought out already...
Perhaps, there is a name and it will be revealed later.