Bunny-Ear Senpai

Bunny-Ear Senpai is the fan name given to a previous member of the Science Club. Her real name has not been revealed yet. She is a year older than Ueno. She is revealed to have been president the year before Ueno in Chapter 82, where she runs last in the relay race, as each club president is required to. In Chapter 94, Ueno and Tanaka refer to her as "last year's President."

Appearance
She has long hair with bunny ears attached to either side of her head. She does not have these in Chapter 60, before Ueno joins the science club. It is confirmed in Chapter 94 that they are an invention of Ueno's.

Her eyes are constantly half-lid.

In a chibi drawing on the back of Volume 5 she is revealed to have black hair and eyes, despite appearing to have lighter hair and eyes than the other characters with black hair/eyes in the manga.

Personality
She is free-spirited, acting on her own whims most of the time. She also has a violent side, imagining Sprin-kun spraying poison, and asking it to fill itself with Tanaka's blood. She is also competitive, having used Ueno's inventions on the former club president to win the relay contest in Chapter 81.

She is the only other member of the Science Club we've seen invent something, though her inventions are low tech compared to Ueno's.

History
The following events are in chronological order, rather than by chapter:

In Chapter 60 she puts up posters for the Science Club, which attract Ueno to the Science Room. She grabs Ueno on her way out and forces her to join the Science Club.

In Chapter 92, she harasses Ueno on a skateboard invention along with Unogawa and Hanamori.

In Chapter 81 she uses Ueno's boosters on the club president's feet to win the relay race. In Chapter 82, she is seen in a flashback having used the shoes to win the next year, too.

In Chapter 70 she tries out Sprin-kun and gets angry with Tanaka. She then makes it try to kill him.

In Chapter 50 she shows Yamashita her cleaning rag invention.

Ueno
She seems to rely on Ueno to make her high-tech inventions. In Chapter 94, we learn that the bunny ears she wears are an invention of Ueno's that allow the wearer to talk to specific people from a distance. This is demonstrated in Chapter 81, though unexplained there. Not much else is known, but they seem to be friendly with each other.

Tanaka
She doesn't seem to like Tanaka, and tries to kill him with Sprin-kun, but they've had minimal interaction so far.

Glasses Senpai
Glasses Senpai follows her around. She will stop Bunny-Ear Senpai from going too far with her malicious thoughts.