In the second season finale, Jeremy (Steven R McQueen) discovered that he was able to see his dead ex-girlfriends Vicki and Anna (Malese Jow).
However, Ewell explained that although Vicki and Anna returned together, they will have different agendas.
"Vicki has a master plan and it's trying to be stopped by Anna," she told TV Guide. "A lot of what Anna says is, 'Beware of Vicki, be careful'."
Ewell added that Vicki will be determined to begin her romance with Jeremy again, even though he also dated Anna and is currently seeing Bonnie (Katerina Graham).
"All Vicki does is toy with people," she said. "She doesn't know any other way. Of course I think ultimately Vicki wants Jeremy to be happy... but Vicki can be selfish and her one and only love is Jeremy so she's going to go for it as much as she can. Vicki's trying pretty hard to get back into his life and I think there will be a lot of success there."
She teased: "They keep hinting towards this huge epic fight between Bonnie, Anna and Vicki. If that were to happen, there would be major powers at stake there because we're talking two vampire ghosts and a witch."
Ewell also explained that Vicki will try to help Matt (Zach Roerig) get closure about her death and will make him aware of her presence by moving things in the house.
However, she added: "I can say I do come back... they've been teasing we don't just remain ghosts."
The Vampire Diaries' executive producer Julie Plec has previously confirmed that Matt will go on "an emotional journey" in future episodes.
The Vampire Diaries will return for its new season on September 15 on The CW.
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Watch a trailer for the third season of The Vampire Diaries below: