The Lamplighters League

Madame Mei is a supporting character that is required to unlock more card slots for the Undrawn Hand as well as raise the level of these cards from 3 to, eventually, 5.

She is recruited by succeeding her recruitment mission which in turn is unlocked from sending an agent on an expedition (the magnifying glass looking missions). The mission will note what type of outcome (special mission or recruitment) it yields.