Jian mother and jitae father were having an affair. And jian was supposedly born out of the affair. The grandfather still considered jitae to be the only legitimate grandson so jian mom killed jitae mother in an attempt to eventually marry jitae dad and making jian a legitimate heir. However we just learned jian bio father is not jitae dad but president Yang. Which means jian and jitae are no longer related at all. They aren't actually half siblings. She was just a greedy whore
I'm also not too sure about him but my guess is that jian mother was using him to keep tabs on jitae. The same way she's paying the house staff to report to her what jitae does. So he wasn't a genuine friend. Just pretending to be one and because he's been involved for a long time he now knows A LOT.
I’m glad that more people are as confused as me when it comes to all the different characters and the plot. When I asked a simple question about Jian’s mom, someone rudely suggested I reread the whole story, when they just could’ve easily been helpful and answered the question. I wish there was a creator’s note with all the character profiles because it gets very confusing after there’s been so many smut heavy chapters and hiatuses :/