Visily Romani is a fictional character in the Ally Carter Teen crime novel Heist Society. The protagonist, Kat, contrives with a family friend to have herself kicked out of prep school in order to save her father who has been wrongly accused of stealing painting from a gangland mobster. Kat has some loyal friends with whose help she will prove her father's innocence.
The real theif is one Visily Romani whose crime name is a "Chelovek Pseudonima," a handy alias available to any thief who wishes to don it. This catch-all pseudonym was a trick old crime families employed to keep big secrets from even other family members. In her pursuit of Visily Romani, Kat and her friends tour the world knowing that the only way to clear Kat's father is to steal the paintings back from Visily Romani.
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According to Uncle Eddie, "he is no one and he is everyone". He is an urban legend among the thieves in this story, that represents anyone who acts with altruism and rights a wrong, by way of (re-)theft using that alias. In this story, it could be Mr. Stein and people who helped him return a portrait of Kat's great-great-grandmother to her. (Hale? Uncle Eddie, himself?) It is never made clear. Maybe in "Uncommon Criminals", it will be made clear.
After Kat is set up for a prank she did not commit by her cousin Gabrielle and Hale, she is kicked out of Colgan. She did not act as Visily Romani this time, but as herself, to save her father.