Aki Kokonoe possesses chest-length ochre hair styled in low twin pigtails, featuring bangs across her forehead and neck-length strands framing either side of her face, along with brown eyes – traits inherited by her daughter Rin. She is Rin's late mother and the older cousin of Reiji Kokonoe.
Pregnant by boyfriend Roku Kamihara, Aki was abandoned when she refused his insistence on an abortion. Kamihara later revealed his fear, confirmed by a doctor, that childbirth endangered Aki's life. Despite this, she chose to carry Rin to term and raise her alone, becoming the family black sheep. Aki held no resentment towards her family or Kamihara, believing forgiveness preferable to anger.
When Rin was five, Aki welcomed her recently orphaned younger cousin Reiji into their home. Though initially mistrustful of adults, Reiji warmed to Aki through her gentle, quirky nature and affection for Rin. Despite their 13-year age difference and being cousins, they eventually entered a relationship after Reiji developed romantic feelings, forming a family unit.
During a routine exam, Aki was diagnosed with Stage 2 lung cancer, complicated by health vulnerabilities stemming from Rin's birth – the original reason for Kamihara's abortion plea. Learning treatment costs could instead support Rin for years, Aki secretly chose to forgo care for her daughter's financial security. She concealed her condition until Reiji discovered it. By then financially stable, Reiji convinced her to seek treatment, but the cancer proved too advanced and aggressive. After two months without improvement, Aki returned home. She hoped to live long enough to see Rin enter grade school but succumbed before then, with Rin remaining at her side until the end.
Aki died over two years prior to the series' main events but remains significant in the memories of Rin and Reiji. Her life choices, sacrifices, and personality continue to shape their actions and emotional lives. Her name "Aki" means "autumn," and her surname "Kokonoe" translates to "ninefold, imperial palace, court." Her blood type is A.