Mukashi No Otoko Tomodachi to Dokyo Wo Hajimetara, Jitsuha Bishojodatta Kyori Kan Ga Ano Koro No Mama De Chikasugiru 1 (Overlap Bunko) [Light Novel]

[Machine Translation] Ke-chan, I'll see you next time" - The promise was never fulfilled, and Keisuke Kirishima was separated from his only best friend, Mar-chan, when he was a child. Ten years later, after his parents remarried, Masaki, aka Mar-chan, grew up to become a lovely, sunny, beautiful girl! While Keisuke, who had always mistaken her for a man, is confused, Masaki is closer to him as a brother and sister than ever before. They play games together, eat together... Not only that, they even tried to take a bath together! Living together, the two of them feel comfortable as if the 10-year gap has never existed. But their minds and bodies are still growing up, and they can't help but be conscious of each other under the same roof...?

「けーちゃん、またあしたね」―その約束は果たされず、幼き日の霧島圭介は唯一の親友だった男友達・まーちゃんと離ればなれになる。そして10年後、親の再婚でやって来たのは―可憐な陽キャ美少女として成長した、まーちゃんこと真咲だった!ずっと男と勘違いしていた圭介が戸惑う一方で、兄妹として昔よりも近い距離で接してくる真咲。一緒にゲーム、一緒にごはん...どころか一緒にお風呂まで入ろうとして!?共に暮らすなかで、10年の空白がなかったかのような居心地の良さを感じる二人。それでも、二人の心と身体は成長していて―ひとつ屋根の下という状況から、どうしてもお互いを意識してしまい...!?