The only requirement is to be level 250, other than that, you don't need anything special...
Scarlett is part of "The Grave Danger" quest.
There is a path you must take in order to gain access to her, and also a chess game.
All of it can be found in tibiawiki's spoiler, here: https://tibia.fandom.com/wiki/Grave_Danger_Quest#The_Grave_on_an_isle_NE_of_Ankrahmun
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I don't recommend doing it alone... Cobras can be very dangerous
The quest is same logic as Falcon Bastion, you have to kill the mini-bosses, 1 by 1 and you will unlock access to Scarlett by doing so.