She is the newest lineup of Misaka Clones introduced in Toaru Majutsu no Index.

Little information is available for these new clones, but it is certain that they are created for a different purpose than their predecessors. This series of clones are made from the "Third Production Plan," which began after Last Order was taken by Accelerator. They are at Level 4, stronger than the normal Level 2 Sisters and the Level 3 Last Order. These clones can fire railguns, though their railgun can only just go past the speed of sound, unlike Mikoto who can make it go past three times the speed of sound. The color of their lightning is purple, unlike Mikoto's which is blue.

One of them is to replace Last Order as the administrator of the Misaka Network, to bring it into a stage above what they are now in order to further Aleister's plan.

Compared to the first 20,001 clones of Misaka Mikoto (all clones except Full-Tuning), Misaka Worst's physical form is a high school student version of the Level 5 Electromaster; it is also pointed out that they are 2-3 centimeters taller than the original.

Their physical features also differ to the first batch of Sisters; they have dark circles around their eyes and has orange irises, compared to the gray irises of the 20000 Misaka Clones and to the original's brown irises. They no longer wear a similar hairpin as the first batch, and they do not wear Tokiwadai Middle School's uniform; instead, they wear a full-body suit. To prevent this series of clones being controlled by Last Order, all of the clones are cut up and have many metal parts embedded in them, which according to the Misaka Worst clone that fought Accelerator was painful beyond belief.

As for their personality, though they have the same appearance as Mikoto, their personality was programmed to be similar to Mugino's.