Edit
One of the many clones of Misaka Mikoto produced as part of the Radio Noise Project, and later repurposed as part of the Level 6 Shift Project to be used as fodder for Accelerator.
This specific Sister is distinguishable for being the only one with expressive emotions - a result of an attempt by Nunotaba Shinobu to humanize the Sisters and stop the Level 6 Shift Project by injecting her with an emotional protocol virus. While it was intended to be spread through the rest of the Sisters through the Misaka Network, Last Order's unconscious actions prevented it, rendering 19090 as an odd clone. She can often be seen blushing or worrying about her figure, usually by lifting her shirt and measuring her waist.
This tag implicates misaka_imouto (learn more).
1