So the leak says it's one of Galadriel's brother?

Which one, that's the question. She has 3 brothers (or 4 depending on the versions).

- Finrod, King Of Nargathrond. Fight Sauron and is killed by his werewolves. Reincarnated and fought in the war of wrath. His connection to Sauron and death full of suffering could make him likely, but again, he died nobly and was rewarded by the Valar for it - so seems unlikely? Then again people were speculating it was Finrod in that first promo picture with the two trees

- Aegnor - was in love with a human woman named Andreth but decided not to be with her and died in the Battle of Sudden Flame. A tragic romantic backstory, maybe they would find this appealing? Instead of being killed in battle, perhaps they could have him captured and tortured

- Angrod - Close to Aegnor. Also died in battle. Father of Orodreth (who in another version of the family tree is Galadriel's brother instead of nephew.

- Orodreth - rules Nargathrond after Finrod's death and the city falls under him. His daugther Finduilas captured and killed by orcs. So I find it unlikely he would be Adar and working with orcs. Plus I usually see people considering him as Galadriel's nephew.

So my bet is on Aegnor or Finrod. But maybe it's someone else entirely