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An aged down version of Shigure Ui, as seen in post #3760868.
It originally started as a throwaway joke over an illustration Ui drew of a younger version of herself, until she lost a bet against Nijisanji's Kenmochi Touya, and was forced to do a stream pretending to be a little girl on-stream using the said illustration as a static avatar on February 2020. It became a running joke on her channel from then on, ultimately debuting a Live2D model for April Fools 2020, and her occasionally doing "Loli Ui" streams with either the static or Live2D avatar, until it got a full 3D model on September 10th, 2023.
She wears a school uniform consisting of a white long-sleeved collared dress shirt with a red bowtie, a blue pleated pinafore dress, white kneehighs and white footwear uwabaki. She also carries a red randoseru with a crime prevention buzzer and has a bandaid on her knee.
Her hairstyle in this form is short hair tied into twintails with swept bangs and pom pom hair ornaments.