its in LN, so in manga its just extra chapter. u can read the novel in strawberry scanlation page i think
The novel has 5 volumes right now(6 in the process), and they explain since the beggining their relationship that started just after the rejection of Yokozawa from Takano. It also has a movie, but in a personal opinion the novel its much better because things are way better explained (and there is more "romantic" stuff if you know what I mean hehe). It's a very enjoyable and easy read :D
When does takufumi first meet his lover. I read takafumis first chapter but it looks like they knew each other before