Sonia Summers is a young female detective, originally stationed in Upper Town but demoted to the 18th branch in Outside due to her serious nature and stubborn commitment to justice. Refusing to conform to Outside's corrupt customs, she persistently pursues what she believes is right, often investigating mysterious cases involving supernatural occurrences. Transferred to the port city of Libra, she encounters its stark division between wealthy Upper Town and crime-ridden Outside. Her investigations frequently involve incidents linked to humanoid youkai, leading her to cross paths with three such entities—Bem, Bela, and Belo—who protect humans from malevolent youkai. Initially perceiving them as threats, she gradually recognizes their protective role despite their monstrous appearances, though their interactions leave her questioning the nature of justice and humanity. Her commitment often results in perilous situations, including being kidnapped and nearly mutated by antagonists. She demonstrates resourcefulness in escaping captivity, such as manipulating a captor's motivations to create distractions. Later, depicted in *BEM: Become Human*, she spends two years searching for Bem, Bela, and Belo after their disappearance. Her investigation leads her to Doracho Chemicals, an island city, where she encounters a man named Belm who resembles Bem but claims no memory of her. This search culminates in a confrontation where she is fatally wounded but revived through Bem's sacrifice. This act transfers his spirit into her body, resulting in a fusion of their beings. Visually, her eyes acquire hellish pupils characteristic of youkai, suggesting she now exists as a hybrid entity carrying Bem's essence and memories.