Lady Proxima is the Grindalid matriarch of the White Worms, voiced by Linda Hunt in Solo: A Star Wars Story (2018). StarWars.com places her precisely: she "rules Corellia's black market from a tangle of sewer tunnels beneath Corellia's Coronet City", where "a briny pool serves as a throne room".
Scrumrats
Her operation runs on children. The databank names them — "the urchins known as scrumrats" — and describes the business model without softening it: she sends them out "to pick pockets and barter stolen goods", feeds them badly, and keeps them in debt. A young Han Solo and Qi'ra are two of them, and the film's opening ten minutes are about the pair trying to buy their way out.
A weakness written into the design
Grindalids are burned by sunlight, which is why Proxima lives in a sewer and why the escape works: Han shatters a window and lets the daylight in. It is a tidy piece of construction — the boss's power is total within her tunnels and evaporates the moment the environment changes, which is as good a summary of gang authority as the film offers.
What she establishes
Corellia in the films had been a name attached to Han and to a make of freighter. Proxima is the first thing that gives it texture: an industrial shipbuilding world with an underclass, a black market and a captive labour supply, which makes Han's obsession with flying away considerably more legible.