With respect to the plot structure of Light in August by William Faulkner, in what ways is Lena a catalyst, "effecting change but itself unchanging"?
Lena Grove is a very simple country girl, but she is relentlessly unchanging in her motivation to find Lucas Burch, aka Lucas Brown, the man who is the father of her unborn child. Lucas leaves her initially because he doesn't want to marry her--athough he has made her believe that he loves her and is only leaving to look for a job so that he will be able to support her and the baby. He would not find himself working with Joe Christmas at the mill, then selling bootleg whiskey with him and finally living with him if he hadn't been fleeing from Lena.
Unfortunately, Lena does not see this quality in him and so he is able to con her into believing that he would marry her and provide for their family. Lucas "liv[ed] on the country, like a locust," waiting to land on any opportunity that he could work to his advantage. Others knew that "he'd be bad fast enough … if he just had somebody to show him how." He is loyal to no one, including Lena, whom he deserts, and Joe [Christmas], whom he turns in for the reward money.
Lena brings out changes in minor characters she meets along the way. Everybody is impressed by her determination and fortitude. As poor as they are, people do what they can to help. Armstid gives her a ride on his wagon and puts her up for the night in his home. His wife goes even further. She gives Lena all of her "eggmoney," i.e., the money she gets from selling the eggs laid by her chickens. Next morning Armstid gives the pregnant girl a ride all the way to Varner's store (a place featured in Faulkner's novel The Hamlet), where he arranges for her to get a wagon-ride all the way to Jefferson.
She rarely has to ask for anything from those she encounters on her incredible journey from Alabama to Mississippi. They can see her condition and understand what she needs. The character Lena influences the most is Byron Bunch. He falls in love with her and helps her in every way he can. His role in the novel is described in eNotes Study Guide:
After Lena comes to town, Bunch proves himself to be an honest and decent man. Initially, he tries to think of ways that he can keep Lena and Brown apart, but his conscience, along with some prodding from Hightower, forces him to place Lena's needs above his own. Even though he is in love with Lena, he tries to reunite her with Brown because Brown is the father of her child. When Brown deserts her again, Byron's protective nature emerges as he demands justice for Lena and challenges Brown to a fight that he knows he will lose. By the end of the novel, he leaves with Lena, taking an active role in plotting the direction of his life.
Lena is finally brought together with the despicable Lucas Burch and gives birth in the cabin he had been sharing with Joe Christmas. (It is because of his care for Lena that Byron gets the defrocked minister Hightower to help deliver the baby.) But Lucas climbs out the window and takes flight again. When Lena takes up her journey, evidently still determined to get Lucas to marry her, Byron goes with her, abandoning his job and his life in Jefferson and perhaps symbolically becoming like the biblical Joseph with his wife Mary and the newborn baby Jesus.
At the very end, Lena says:
"My, my. A body does get around. Here we aint been coming from Alabama but two montns, and now it's already Tennessee."