Akane and Aoi are siblings, 11 months apart in age. Although Aoi is a siscon, he is still better than Akane in track and field... The one who comforts her frustration at being overtaken by her brother is Kamiwazumi-kun, who is famously quiet!
(Source: Akita Shoten, translated)
— Description from AniList community (anilist.co).
