Leona was born in an unnamed village, residing with her family. When she was a child, Goenitz arrived at her home and demanded to talk with Gaidel, the leader of the village. He requested that Gaidel join him in their duties to serve Orochi, as everyone in the village were Orochi bloodline members. However, Gaidel refused as he and his family liked peace and not the violent tendencies created by the Orochi blood. Enraged, Goenitz leaves, but not before smiling towards Leona. The same night, Leona underwent the Riot of the Blood and slaughtered everyone in the village. Goenitz, pleased by the results, puts her blood lust to rest for another time "when he needs her again". Dazed and shocked, she loses her memories and wanders aimlessly until she is found and later adopted by Heidern. Surprised by her natural fighting ability, he decides to raise her as a warrior under his command.
In 1996, Leona is in her late teens and a capable mercenary. She receives her first official mission: she must take Heidern's place in the Ikari Team for the year's King of Fighters tournament. Her partners were Heidern's favorite mercenaries: Ralf and Clark. Their mission was to investigate Geese Howard, the South Town kingpin of crime, who had entered the tournament in search of an unknown goal. Knowing that anything Geese may be after could only mean trouble, the new Ikari Team began their mission. During the events of the '96 finals, Goenitz confronts her and reminds her of her Orochi lineage, causing her to worry about the future.
Her troubled state carried over to the battlefield, which eventually lead to her and her partners' suspension from military service. Wanting to cheer her up, Ralf enlists them again in the next year's tournament. She eventually remembers her past fully during the events of King of Fighters '97. Towards the tournament finals, Leona, along with Iori, suddenly went into the Riot of the Blood, induced by Orochi. By the tournament's end, she is successfully subdued and Orochi is sealed away again. Even so, Leona regrets her actions and attempts to end her life. Ralf stops her, and encourages her to continue living.
Leona stays as a member of Ikari team for each and every King of Fighters tournaments during the NESTS saga. It was assumed she partook in her normal duties with Heidern during this time frame.
In 2003, at the end of the tournament, Orochi's presence was getting stronger, as the seal was broken again. She entered the Riot of the Blood again and attacked Ralf and Clark, badly injuring them. She was pulled out of the team by Heidern in The King of Fighters XI due to the emotional trauma she suffered after the incident. Overcoming the shock, she is designated to join the KOF XIII tournament with her old teammates, determined to live with whatever flaws the Orochi blood invokes. After the tournament's climax, she loses consciousness and sleeps for several days. Her teammates were worried for her and stay beside her until she awakens. Comforted by their concerns, she laughs for the first time in the series.
She fights the same way as Heidern did, except she can now create a sphere that cuts everything around it as she screams. She can also throw her earrings as explosives. She has then received more explosive attacks as the series continues.
She made a guest appearance in Metal Slug XX as a Downloadable Content to contrast with the playable Ikari characters.