Deidre is a mysterious mage with an oddly masochistic bent. This leads Lily to dub her "the dirty old lady," much to her displeasure.
She is fascinated with Emilio's power, and challenges him as he descends into the labyrinth, hoping to see that power in action. She loses, and is shocked when he confirms his victory by raping her. This shock may be feigned, however, as she teleports to safety the moment he relaxes his guard. Deirdre seems not to hold a grudge for this incident; later, she assists Emilio by giving him the tools necessary to subdue the vastly-more-powerful Estelle.
Deidre's true motivation is a desire to see a pure world of chaos, born of the neverending conflict between humankind and demons. For this, she wants to see the Overlord revived; however, she believes that Emilio has been "tainted" by his human guise. As he closes in on his sealed body, she strikes, attempting to seize the demonic power for herself.
She rarely takes action herself, preferring to let others like Emilio, Sylphine, or Estelle do her work for her.
The partial walkthrough in the pre-order extra disc also refers to Deidre as hentai-san(ヘンタイさん).