theories in Death fan community

I believe that this animated series will tell about Mateo, who even though he is the protagonist, we know nothing about him

New theory, Mateo will be arrested by Severian, at the end of chapter 3, Mateo is arrested in the castle prison, we don't know who arrested him. He will run away and meet Relicta, he will talk to her and she will join him.

Well, my theory is as follows, Mateo will kill Relicta, to get out of the dead world