Introduced in both the third volume of the novel and manga respectively, and episode ten of the anime, Tōma meets the younger "twin sister" of Mikoto whom he calls Misaka's sister (御坂妹, Misaka imōto?), who seems rather quiet and emotionless. Her voice is rather monotone and machine-like, and her speech pattern is unique; as she speaks, she adds a description of herself speaking before in third person. Despite this, she has a soft spot for cats. Mikoto seems to dislike and worry about her for some reason and later events reveal why.

(Source: AnimeNepal)