Spiller is a young Borrower who lives independently in the wilderness near the human house inhabited by Arrietty's family. His dark skin features distinctive white stripes on both cheeks, complemented by uneven, spiky hair. Slightly muscular with a sharp facial structure, he stands shorter than Arrietty. He favors practical survival attire, including a brown fur coat doubling as a makeshift glider, and wields a bow and arrows against threats.

Accustomed to solitude in nature, Spiller exhibits exceptional self-reliance and resourcefulness. His outdoor adaptation includes hunting crickets for sustenance and navigating via streams and vegetation. Agile in movement, he traverses terrain through climbing and leaping. Personality-wise, he communicates minimally, prioritizing action over words while maintaining a calm, observant demeanor. He displays practical problem-solving skills and loyalty toward fellow Borrowers, though wary of human contact initially.

Spiller's narrative role initiates when he discovers Arrietty's injured father, Pod, during a borrowing mission. He aids Pod's return home and later proposes relocation sites using his knowledge of safe habitats beyond the house—a response grounded in Borrower survival protocols after human detection. When the family faces evacuation due to the housekeeper's persecution, he coordinates their escape down a stream via floating teapot transportation. He then guides Arrietty and her parents toward a new settlement, implying ties to other Borrower communities.

His interactions with Arrietty suggest developing respect or affection, though he remains reserved emotionally. His actions consistently prioritize Borrower safety and continuity, underscoring themes of resilience amid their dwindling population.