Hester van Zandt is the big momma of the group of people protesting the Swannekke reactor. She acts as a guiding star for Luisa—which is appropriate, given that the name Hester means "star." Aside from furthering the plot, she drops a heaping gob of symbolism into the book near the end of Part 9.
Right before Margo Roker wakes up from her stroke, Hester reads Emerson's poem "Brahma" aloud to her. Margo Roker is the woman Bill Smoke beat into a coma—under orders from Seaboard, so that they could rack up her hospital bills and buy her land, thereby ceasing the protests. Despicable doesn't even begin to describe it.
One interpretation of the Emerson quote is that we humans cannot find fulfillment alone; we can only be content when we realize we are part of a larger thing, and act accordingly (source). Before Hester can finish the poem, though, Margo Roker wakes up. Why? Because Smoke is dead, Luisa has exposed Seaboard, and all is right with the world. Luisa did the right thing, and Margo Roker can safely return to a slightly less dangerous world. Yeah, that fits pretty well within the themes of this book. Thanks, Hester.