Lavenza is a mysterious guiding voice for the protagonist, used to urge him to take action in the real world. Until much later in the series, she manifests only as a spectral butterfly.

Her voice should be in a high range. She should sound angelic and otherworldly, a guardian spirit to guide the protagonist.

You can look at the game's dub actress, Carrie Keranen, for reference.

(Since Lavenza, Caroline and Justine are all technically one being it is likely that I will cast the same person for all three, though I may make an exception if another voice just happens to work perfectly for any of them individually.)