Location: West Wing, fixed location
Rescue Reward: regeneration potion
From the Manual: Melissa is a London medium and adviser to your mother since your grandmother died. During this time Melissa has become a close friend, and has been asked to join the family to Romania to support your mother.
Like the other West Wing hostages, Melissa's location is randomly determined when you begin a new game, but she will always appear in the same room even if you reload from a previous save game. Melissa is probably the most difficult relative to rescue, because her death is "hardwired" to 18th minute upon entering the West Wing. Because of the game's random placement, it's entirely possible she'll be placed on the 4th floor, the very last floor you will be able to access in the West Wing.
If you're a completionist and want to rescue all your relatives, make a hard save just before entering the West Wing for the first time. If she ends up dying before you can rescue her, reload from that save and rush through the West Wing only doing the essential tasks until you manage to locate her. If you're not extremely determined on rescuing all family members, don't let her death upset you. The 6 minute regeneration potion she rewards you with is nice and helps in the final battle, but it's not absolutely vital. Besides, you can unlock her trunk on your own using either Wilfred's skeleton key or Lord Belmore's master key.