Kuroyoru Umidori is a mysterious girl who apparently is connected to the dark side of Academy City. She poises as an antagonist in Shinyaku Toaru Majutsu no Index Light Novel Volume 01; she is a member of the mysterious organization known as "Freshmen". She looks down others as evident with her conversation with Kinuhata Saiai and treats anyone as disposable things. Accelerator remarked that talking to her was like having a conversation with his "old" self. She has a liking to stuffed doll dolphins and is always seen carrying one.

Spoiler

She is one of the subjects from the Dark May Project, just like Kinuhata. However Kinuhata Saiai was treated as the star of the project, the "honor student" who perfectly followed the program, while Kuroyoru herself was thought of a delinquent whose rebellious attitude pushed the program to bankruptcy . This fact caused a slight fixation from Umidori to Saiai when they meet in New Testament. As to throw salt in the injury, during New Testament the Accelerator pointed out that Saiai was better suited for his cognitive patterns than her. It should be noted that her rude demeanor and cruel personality are a side effect of the project, even to the point where her speech pattern is influenced by the data from the Accelerator.

(Source: Toaru Majutsu no Index Wiki)