The author has been setting up hints throughout the story using visual and dialogue ques. Taku requested Haesoo to write about him after reading the story that had no ending about a stepbrother who becomes a lover and kept asking about it. Taku didn't know it was Joowon until Leo showed him the picture of them together and told him it was his stepbrother. When Taku drove Haesoo home he glanced at Joowon before Haesoo had a chance to notice him. Taku just happens to request a shoot with Joowon and, oh yeah, fucking Haesoo. There are others, but the most standout ones are the ones in his chapter. The panel zooms in on Taku's eyes when he specifically says his theme of "heartbreak" after pushing the photoshoot up with Joowon.
Are you blind?

I love Taku and I ship him with Haesoo, but Taku is shady as fuck. All ya'll hatin on Joowon, thinking he's the worst, but Taku be over here plotting.