Lovely, wry, playful, inaccessible Lux is the creature of each narrator's dream. The boys watch from school windows as she laughs her beautiful laugh at other boys, and from their treehouse as she responds to the house arrest by having sex on her roof with a stream of faceless men. Both her nascent sexuality before Homecoming and her explicitly sexual escapades after impress the neighborhood boys, who claim that all subsequent women in their lives have, in moments of passion, taken Lux's face. To the boys, Lux's seemingly intuitive knowledge of sex suggests an intimacy with the secrets of the world and concomitantly, a familiarity with death. Yet neither the boys nor Lux's partners are able to fathom the depths of her mind, to determine her thoughts on Cecilia's death, or even her motives for anonymous intercourse. Though her partners find her magnificent—a carnal angel—they report that Lux often seems bored by sex, picking zits on her partner's back or looking off into the distance. Yet her presence and sense of purpose lead the men to feel as if they have been insignificant pawns in Lux's higher plan, echoing the boys' feelings on the night of the June fifteen suicides. Furthermore, her insistence upon the sexual act itself, in the winter theater of the Lisbon roof, seems to suggest a will toward both performance and self-destruction, yet these are precisely the characteristics that the narrators most want from Lux. Thus, it is difficult for the reader to discern Lux's true intentions, as they are observed by the boys' desire.
Narratively, the first signs of Lux appear when Peter Sissen, having dinner at the Lisbon house, passes her bedroom en route to the bathroom and sees her bra hung carelessly from a crucifix. Peter emerges from the bathroom and is confronted by Lux, her hair let down, waiting to get a tampon. This twofold evidence of Lux's budding femininity and sexuality impresses Peter and the boys. Likewise, the tantalizing image of her brassiere foreshadows Lux's subsequent promiscuity in the face of Mrs. Lisbon's strict religious regime. Lux next appears at Cecilia's party, where the narrators realize she is the only Lisbon sister truly as beautiful and mischievous as they had imagined. For some time after, Lux's character retreats behind a series of constraints on her femininity. For example, Mrs. Lisbon wipes off Lux's lipstick or sends her inside to change into a less revealing top. In rebellion against these restrictions, the boys see Lux accepting a ride on a motorcycle, and laughing outside the high school with a delinquent boy. But by the middle of Chapter Three, with Cecilia dead, Bonnie, Mary and Therese are given only collective or passing mention, while Lux emerges as a heroine of singular importance.
Like Trip, whose silence and stature excludes him from the group of neighborhood boys, Lux rarely interacts with girls besides her sisters. Occasionally she will ditch gym to smoke with a friend, but even these few moments cease as the Lisbon sisters' tragedy mounts and the community begins to leave them alone. Yet even among her sisters Lux is strikingly individual, venturing out alone into the risky territories of sex, cigarettes, alcohol, and love. Lux's access to the world of men leaves her as a protective intermediary between her sisters and the neighborhood boys—she waits outside for the boys' car at Homecoming and later stalls the boys while her sisters commit suicide. Lux exists in the threshold between masculine and feminine worlds, Lisbons and outsiders, adulthood and adolescence, life and death, virginity and knowledge, and seems finally to belong to neither extreme even as she seems to alternately typify them. She is impossible to pin down, and wriggles cleverly out from under labels or uses them for her own purposes. Lux's elaborately faked 'burst appendix,' a cover for a pregnancy test, as well as her numerous forged excuse notes at school, indicate her command of charade, leaving both the reader and the boys to wonder about her ultimate motives. Yet she is far more than a trickster, her games suggest an imaginative response to impossibly restrictive rules rather than dishonesty or fraud. She is the most ambiguous and therefore the most human of the sisters, actively fighting to live outside of the rules that restrain her. Lux is ultimately abandoned both by the awed boys and by manly Trip, though they will insist they loved her.