AKA as Misaka Imōto, she is the most prominent clone that is most featured in the series and is known for having a soft spot for cats, while she herself has a pet black kitten. She was the second sister clone that Touma met.

Introduced in the third story arc of the series, they are Mikoto Misaka's clones, which Touma refers to as Misaka's little sisters (御坂妹; Misaka imōto).

Unlike Mikoto, they seem rather quiet and emotionless, their voice is rather monotonous and computer-like and they talk in third person by adding a description of themselves after they finish talking. Also, they wear a special set of goggles that enables them to see electron beams and magnetic field lines for the reason that, unlike Mikoto, they do not have that ability.

Even though they have a weaker variant of Mikoto's electric abilities (Level 3), the Sisters' minds are connected by a special psychic ability called Radio Noise (欠陥電気 Kekkan Denki, "Flawed Electric"), which enables their brainwaves to become electric signals; this allows them to share their thoughts and memories within their communication network, also known as the Misaka Network. Being rapidly manufactured clones, the Sisters rarely change their facial expression because they have not learned how to. Due to minds having connected to the Misaka Network, most of the Sisters never considered themselves human, yet some of them had slight difference in personalities.

Originally, it was intended that the Sisters were created as efficient Level 5 combat clones for the Military under Project Radio Noise, but the project was halted when the Sisters were revealed to be weaker and were only Level 2 and 3. The Sisters were later used in another project called "Level 6 Shift", where about 20,000 clones were created in total and were used to be killed by the participant who might become the first Level 6 psychic, Accelerator. By the time of third novel, 10,031 were killed in the experiment before it was finally stopped by Tōma and Mikoto. After the Level 6 Shift experiment fails, all 9969 surviving Sisters are ordered back to research centers around the world to have "readjustments" so that they will have normal life spans. The Sisters later began showing their own unique personalities to differentiate themselves after learning how to 'disconnect' from the Network and keeping their own private memories inaccessible to the others in the Network.

Source:Wikipedia