WTF is a Misaka Worst? Was she left in the oven for too long?
She's a clone from the "Third Season" Project (her showing up for the first time in Index III is totally coincidental), more powerful than the Sisters but not really as strong as Mikoto. Also has an older outward appearance complete with a proper chest.
WTF is a Misaka Worst? Was she left in the oven for too long?
What LegendRazgriz said. Also, just like Last Order, she's the only clone of her type and is a pillar for the Misaka Network. The difference is that unlike Last Order, who is the pillar that acts as a command center and fail-safe for the Misaka Network, Misaka Worst is the pillar that takes on and absorbs all the negative emotions. Sadness, anger, hate, jealousy, and so on. That's why none of the Misaka Sisters feel negative emotions. Hence the name Misaka "Worst."