So, a six eyed butterfly demon has got a divination artifact (we still don't know, what for, so the "real" Li Jie is 100% replaced by him by the end of ep 19). He tries to find out something and spends his lifespan on it. The divination says that it will be hard to get something back and it will be hard to find it again (an English version says so, at least).
UPD: In Mandarin it's 难再回难再寻, so it's "quite impossible to get back and to find").
I doubt he spends his lifespan to find out something for Evil Spirit, so he likely tries to find his brother.
In prison, the butterfly tells Li Jie he is just a leftover of the real Yuan Wuhuo (and he believes it himself, of course), but the director shows us this two times (in ep 18 and in ep 19):
He shows us TWO brothers and then matches their faces to Li Jie and the butterfly's faces. And those are two different faces!
In ep 18, Jiu Ying said he sent "him" to devour Yuan Wuhuo, but a few eps before Li Jie was surprised that "a six eyed butterfly demon" is actually two demons, so maybe Jiu Ying didn't mention he sent only one of them. It wasn't shown either, who wrapped the real Yuan Wuhuo in that cocoon, so maybe butterflies did it together. It also strange that the butterfly demon, who believed they are loving and inseparable siblings, didn't look for his brother. And, moreover, if there was only one butterfly in that cocoon, why his unchanged brother didn't look for him?
As for now, it seems to me so:
A butterfly demon couldn't find his brother after rebirth and spent his health and powers on divination to find him, believing amnesiac Li Jie has nothing to do with him because LJ has no demonic power. But Li Jie IS his brother. (That's why a double-petaled hunter said to Li Jie that it's only matters where he is moving now, not who he was before). To get new healthy "skin", the butterfly unknowingly kills his brother, whom he tried to find so hard that he destroyed his own previous "skin". Sounds very GJM-like, as for me.
UPD: I rewatched ep 15. WZQ casually says that it's a butterfly demon in the illusion with Li Jie, and the illusion belongs to brothers who witness it for each other. As we know now, this butterfly demon is one of those two butterfly demons from the past, so Li Jie should be his butterfly brother to witness the illusion for him!
UPD 2: I rewatched (like for the thousandth time) ep 18. When Li Jie says he remembers only that he has got a didi, there are two butterfly brothers on screen in that moment to prevent a viewer from believing it‘s still about the real Yuan Wuhuo and his didi! It's them, my butterflies, 100%!
UPD3: As for ep 15, you should never say to your older selves 和你们不同, 我们 可是 亲密无间, 相亲相爱的兄弟 out of the blue during your villain speech, because the next moment you could become 反目成仇, 分道扬镳 and 一刀两断.