Mei Zhang is a young Chinese girl and a known "fortune teller" as well doing spells. According to Thomas Shelby the washer women believe Mei Zhang is a witch. She is the daughter of Mr. Zhang. Who owns a whore house as well a dry cleaners, that is visited often by Thomas Shelby, and others during season 1.
Mei supposedly puts a spell on Thomas Shelby's horse, the "powder trick" according to Arthur Shelby. The horse, Monahan boy, wins two races, and truly kicks off the Shelby betting business. But looses the third race.
Mei is first seen holding a crying baby. But it's unknown if the baby is hers or a younger sibling. She hands off the child, to a young Chinese girl washing clothes to breastfeed from. While her exact age is unknown, by season 5 she would be a grown woman.