Anko Koshi is fifteen years old and a first-year student at Hino Minami High School, the younger sister of Torako Koshi by approximately two years. She transferred to this high school after attending an unnamed middle school. She and Torako share a home with separate bedrooms. She has shoulder-length deep purple hair. Her eyes usually have dark pupils and irises but shift into a distinct target pattern with glowing red centers during intense emotional states, termed her "killer mode." Anko typically wears a gray fleece sailor uniform with a yellow scarf, though she sometimes appears in a navy overall-like uniform with a bow. Notably, she uses Torako's old first-year high school uniform instead of acquiring her own. Standing 155 cm tall, her physical features closely mirror her sister's. Anko exhibits an extreme, obsessive devotion to Torako, striving to remain constantly by her side. This fixation drives hostility towards anyone perceived as a threat to their bond, including threats or attempts to physically eliminate rivals. She declares the world needs only herself and Torako, viewing all others as enemies. This possessiveness initially led her to sabotage the Deer Club premises and target Noko Shikanoko, employing kunai throwing and trap-setting skills. An attack where a kunai she threw nearly struck Torako but was intercepted by Noko—demonstrating Noko's importance to Torako—forced Anko to reluctantly accept Noko's presence. This resulted in an uneasy truce and eventual cooperation within the Deer Club, though her primary allegiance remains solely to Torako. Outside contexts involving her sister, Anko displays a reasonable, calm, and capable demeanor, even showing kindness. She prepares rice balls for Meme Bashame and provides chalk for the creature Tsuchi after field work. She demonstrates intelligence, answering complex questions succinctly and deducing biological phenomena like Noko's molting. Academically gifted like Torako, she taught herself practical skills including trap construction and weapon use. Her abilities include adept kunai handling, strategic trap setting, hosting events like the school sports festival, and cooking—she routinely prepares praised bento for Torako due to Torako's poor culinary skills. She also built an observation room with an AI beneath the Deer Club space. Within the Deer Club, Anko participates as Torako's self-appointed "official helper and personal deer," largely ignoring directives from other members unless they support Torako. Despite this, she possesses substantial knowledge about deer and club activities. Her relationships with other members are utilitarian or tolerant; she interacts politely with Bashame and acknowledges Noko's role for Torako but maintains emotional distance. When challenged by Student Council members like Neko Nekoyamada, she employs intimidation to protect her sister's status. Key developments include her initial campaign against Noko culminating in the kunai incident and reconciliation; her enrollment at Hino Minami High ensuring proximity to Torako; and her participation in club activities like playing "Game of Deer Life" or assisting during the sports festival, showcasing integration while prioritizing Torako's attention. Actions like patching Noko's jacket with her sewing supplies or rejecting Bashame's requests illustrate her selective engagement filtered through her sister-centric worldview. A notable physical trait is her cat tongue, causing difficulty with hot foods. Her nickname "Koshian" references the Japanese red bean paste dessert "koshi anko," playing on her full name.