Bonnie's the middle daughter, and like many middle children, she doesn't get all that much attention in the novel. She's religious like her mother, carrying a rosary and burning votive candles in her room after Cecilia's death. Here's our first glimpse of her:
We saw at once that Bonnie, who introduced herself now as Bonaventure, had the sallow complexion and sharp nose of a nun. Her eyes watered and she was a foot taller than any of her sisters, mostly because of the length of her neck which would one day hang from the end of a rope. (1.42)
Well, that's a shocking introduction.
Bonnie's quiet and shy, not much interested in having a social life. She's the only one who doesn't secretly flirt with Peter Sisson when he comes over to dinner at their house; she just said grace and ate her food. The boys describe her as "skittish" (3.116) and Lux thinks she's a "goody-goody" (3.175). She's talkative enough on the group date to Homecoming, but when Lux and Trip sneak away to make out under the bleachers, Bonnie follows them. The boys think she was afraid of being left alone (3.166). At first, she refuses the peach schnapps that her date tries to get her to drink, but after he tricks her into it, she loosens up:
Years later, Joe Hill Conley boasted that he could analyze a woman's emotional makeup by the taste of her mouth, and insisted he'd stumbled on this insight that night under the bleachers with Bonnie. He could sense her whole being through the kiss, he said, as though her soul escaped through her lips […] acidic with woe. Sometimes her lips grew strangely cold, and, peeking, he saw that she kissed with her frightened eyes wide open. (3.176)
She lets Joe Hill Conley kiss her again when he drops her off at home after the dance, and even asks him if he'll call her. There was something transformative for her about that evening. Of course, she did ask him if he believed in God, and he said yes. We'd guess that was a prerequisite for Bonnie. Anyway, he doesn't call.
After the lockdown, the boys see Bonnie gradually waste away. The boys would catch glimpses of her in a baggy shirt covered with feathers that smelled like wet birds. She was always hugging an old pillow, and seemed to have trouble walking. Uncle Tucker told the boys that he was worried about her losing weight:
[…] he watched as with uncoordinated movements, Bonnie Lisbon came down the two front steps, and […] at the site of her sister's death, began to say the rosary. Holding the pillow in one hand, she told her beads with the other, making sure to finish before the first house light came on down the block and the neighborhood awoke. (4.64)
The boys don't know if her emaciation was from starvation or asceticism, but Uncle Tucker thought she looked peaceful as she came out every morning to say the rosary. Religion was Bonnie's way of coping with Cecilia's death; she kept a little shrine to her in her bedroom.
On the night of the "escape", the boys see Bonnie in her bedroom packing her clothes and books in a suitcase. Here's our last, horrifying look at Bonnie:
Hanging down amid the half-deflated balloons were the two brown-and-white husks of Bonnie's saddle shoes. She had tied the rope to the same beam as the decorations. […] Above him, in a pink dress, Bonnie looked clean and festive, like a piñata. It took a minute to sink in. We gazed up at Bonnie, her spindly legs in their white confirmation stockings […]. She spun slowly, and at one point her face broke out of the seaweed of balloons, showing us the reality of the death she had chosen. It was a world of blackening eye sockets, blood pooling in lower extremities, stiffening joints. (4.208)
That's the last we see of poor Bonnie. The boys run screaming from the house. But for a split second they realize that seeing Bonnie hanging there has taught them something important: they had never really known the Lisbon girls.