Justine is one half of the "Twin Wardens" the young denizens of the velvet room instructed to assist Ren in his "rehabilitation" by Igor.

Her voice should be in a high-mid range. She is always gracefully calm unlike her sister and speaks with a voice almost devoid of emotion. Essentially she is the cold, logical half of the twins.

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.)