There have been theories on this before, I personally think it's Sage. This is all strictly conjecture, no real evidence has dropped in the first 6 episodes but . . .
1. Sage/Jade = Green
2. Rooster post, Sage tells Tybalt to 'give it a rest' when he's stalking Jill. Why would she even post on that thread? It sounds like something you'd say to a sibling embarrassing themselves, and you by association.
3. At the Prep party everyone assumes that the Father Sage is avoiding is man bun guy, but what if it's Stephen? The description fits. Also Sage doesn't mention that her Mother is present because Jade wasn't supposed to be there, she had plans with Cathy that she changes at the last minute because Cathy is at Bella's.
4. Or what if Man Bun Guy is Sage's Father, and Jade's first husband?
5. The Dance. When Sage and the MC dance at the DIK party then almost fuck in public, Jacob cockblocks MC. In the dialogue he warns him that if he fucks Sage in public then everyone will be talking about it the next morning. The first consequence of that is that Chad will kill him. The second consequence get's cut off but what if that consequence is that he fucked Sage Burke in public?
6. Sage's influence. When Chad get's kicked of the Alpha's Dawe's first response is to ask Sage's help. In the subsequent conversation in Sage's room Chad says 'they'll listen to you'. Why? Could it be because she's their daughter? Also in the counselors office, when Jade, Cathy, and Stephen are arguing about the approach to take on Chad's punishment Stephen says words to the effect of 'Come on, this is Chad we're talking about'. Stephen later says to Chad ' I know you're not the kind of guy to beat up another guy for no reason' How does he know him? Could it be that's because Chad has been dating his daughter for 2 years?
7. Finally, the things Sage says about her family not showing their emotions is consistent with what we know of Jade and Stephen. Jade talks about how she has bottled up her rage at Stephen's cheating. If she's suppressing her rage, her other emotions aren't being given healthy release either.