Well then, it's known Joah is one of the most confusing character in Zektbach. After unable to bear with one of my favorite characters' death (Lien) I formed a based theory why Lien is Joah, so I will share it here.
1. Starting the Obvious, they look the same. However, we need to point out that in ZETA's video we see that that "crown" Lien has appears as he awakens as divine being, so how Joah has it too? It's a marking of the Rufina genes Lien has.
2. when Lien was looking on at Joah's memo, he sees a really hard puzzle he can't solve. Now, we musn't forget Lien is transcended and the best mathematician in the world, so there is no one except HIMSELF (in the future) that can create a puzzle he can't solve. and of course the fact he also thinks that Joah sounds familiar.
3. I think that in that day Lien didn't really died, don't forget he hears "Joah..is that...you?" that's because L'erisia waited for HIM to come. Lien is the only one that can search for the Risstacia answers, so she wants him by her side, the risstacia side. I mean, he might become the Risstacia's worst enemy by learning its origins after all, maybe how to shut it down, or stop it.
4. in ZETA you have a part like that: "My guided butterfly, you're so intellegent.."
well, who grows wings in Ristaccia clip and 'Guiding' L'erisia? ( Oh Lien you're such a pimp)
5. also:
"After knowing that,you still hold out some hope? Foolishness.
So,I decided to say "farewell" to this hollow world."
"Foolisness", well, Lien IS the smartest, everybody will be stupid for him. and he did say farewell to this world, everyone is sure he's dead.
6.
"We were trapped in a dead end.
It looks like the great maze won't easily accept us.
When did I realize that?
Well,I had already realized it,long long ago."
he realized it when he "died" and became L'erisia's- the Risstacia's puppet, and when he's like that she can make sure he won't search about the Ristaccia, they're always together. He's trapped.
7. In more of Joah's memos, he talks once about ZETA. How can Joah know about Zeta? I thought it was a very hidden path, and that Sigma was sleeping until Lien arrived there.
I think I had more points I dont' remember right now, but these are the main ones I've got. I will add more in case I will remember.
What do you think about it?