This essay explores representations of Indigenous girlhood and ecological crisis in Rebecca Roanhorse’s Sixth World novel series, consisting of Trail of Lightning (2018) and Storm of Locusts (2019). Focusing on the work of an Ohkay Owingeh and African American writer of speculative fiction, I argue that Sixth World portrays its young female protagonist Maggie’s coming-of-age in direct relation to her post-apocalyptic environment, both as a trauma-survivor and an agent of hope, or “Chíníbaá, ... girl who comes out fighting” (Trail 110). By examining Maggie’s liminal and ambiguous subject position as a self-isolated young female “monsterslayer”—which complicates the categories among gods, monsters, and humans after the climate apocalypse—I argue that the series explores the continuing presence of settler colonialism, extractive capitalism, and heteropatriarchal structures that have defined and disrupted both girl identities and the nonhuman environment. Through Maggie’s journey of reclaiming connection and self-awareness, the narrative also navigates possibilities of decoloniality, kinship, and healing within an agentic nonhuman environment. This discussion employs a decolonial and ecofeminist lens and situates the novel series within the broader contexts of Native Apocalypse and Indigenous futurisms, while highlighting its contributions to contemporary environmental and girlhood scholarship. Ultimately, by examining how two crisis configurations, namely of disrupted girlhoods and environments, intersect with the Indigenous apocalyptic in-between in the series, this essay explores Indigenous futuristic and decolonial ethnospeculative aesthetics in addressing interconnected issues of girlhood and the nonhuman environment in contemporary American speculative literature.