Katarina is an anime-only character. She was an Interpol officer and a friend of Minako Aino who met her as Sailor V when she followed a Youma to London. She is one of the few civilians that are aware of a Sailor Guardians' identity.
Six months prior to the events of episode 42, Minako had gone to London after hearing reports of Dark Kingdom activity there, and it was in London that she met Katarina. Katarina was chasing a seemingly normal criminal through the streets when he took a young girl hostage and suddenly transformed into a youma. Sailor V quickly arrived on the scene and destroyed the youma, freeing the little girl. After this initial encounter, Katarina and Minako became best friends and treated each other like sisters. Minako even shared her identity as Sailor V with her and introduced her love interest, Alan. The three went on many outings together, though both Alan and Katarina were unaware of Minako's feelings towards the man.
Six months later, during episode 42, Kunzite had not found out about Sailor V's friendship with Katarina, but also her identity. He hoped to brainwash the woman and use her in order to find out the true identities of the Sailor Senshi. His plan did not work, however; Minako was reluctant to introduce her former friend to the other Sailor Senshi. Kunzite caused Katarina to transform into a youma. After learning Minako's tragic past, Sailor Moon wanted to destroy her in her youma form, believing she stole Alan away from her. Sailor Venus then tells her that she's wrong and that Katarina was the woman Alan loved. The only reason why she was brainwashed was that the Dark Kingdom found out about her real identity while she was Sailor V and used her to find out more about the other Sailor Senshi. Sailor Moon is finally convinced and heals Katarina from her youma form. After the experience, Minako was able to forgive her friend and realize that things were as they should be: Alan was with the one he truly loved, Katarina, and they were happy together.