KATARINA:

Katarina appeared only in episode 042 of the anime. She was a young Interpol police officer who lived in England. Six months prior to the events of episode 042, 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.

One day, while Katarina and Sailor V were investigating at a harbor warehouse, a criminal threw a hand grenade at Sailor V and blew up the building. Katarina believed that Sailor V died in the fire, so she sought comfort in the arms of Alan, who had arrived on the scene immediately after the blast. In fact, Sailor V had not died: she had managed to escape to a nearby alley. When she saw her two friends embracing, she realized that they were in love. Minako never told her friends that she was alive, and she quietly left England, thinking that it would somehow be better for the couple. Six months later (during episode 042), Katarina found out that Minako was not actually dead; she was very much alive, in Japan. Kunzite soon learned about the friendship between the two girls, and hoped to exploit it to learn the identities of the other Sailor Senshi. Katarina traveled to Japan and asked Minako to tell her the identities of the other girls under the pretense of wanting to help them. The attempt proved unsuccessful because Minako refused, so Kunzite used his power to cause Katarina to transform into the youma Papillon.

Papillon immediately attacked Minako, who transformed into Sailor Venus in an attempt to defend herself. Unfortunately, Papillon remembered all of her strengths and weaknesses and was able to incapacitate her with an attack. Sailor Venus ultimately recognized that Papillon was really Katarina because she was wearing a locket that Minako gave her for her birthday. Usagi, Luna, and Artemis soon appeared and took Sailor Venus to safety aboard a luxury liner. Papillon followed them, but Usagi transformed into Sailor Moon and used Moon Tiara Action to stun the youma. She was determined to get revenge on the youma for having broken Minako's heart. Sailor Venus intervened and begged Sailor Moon not to attack, saying, "Please save her! She was turned into a youma because of me. She knew I was a Sailor Senshi; that's why the Dark Kingdom used her. None of this is her fault!"

Sailor Moon ultimately realized that this was the right thing to do, so she used Moon Healing Escalation to heal the youma, who then transformed back into Katarina. After recovering, Katarina hugged Sailor Venus and apologized for the pain she caused, and said that she never suspected that Minako had feelings for Alan. After the experience, Minako was able to forgive her friend and realize that things were as the should be: Alan was with the one he truly loved, Katarina, and they were happy together.

Katarina's seiyuu was Yuko Mita.

See also:
Episode 042 - Sailor Venus' Past, Minako's Blighted Love
Alan
Papillon
Sailor V
"The London Youma"
Yuko Mita