In God of War Ragnarök, the disguised character is revealed to be Odin.
Odin visits Kratos and Atreus at their home while presenting himself under the name Týr. He claims to want peace and access to Atreus's knowledge, but the deception allows him to observe the pair and manipulate events around Ragnarök.
The real Týr is imprisoned elsewhere, and the discovery of that imprisonment exposes Odin's disguise. Odin's search for knowledge and control drives much of the conflict in the game, including his interest in the giant's shrine and the mysterious mask.
The reveal is easy to confuse with Thor's role because Thor also serves Odin during the story. Thor is Odin's son and a major adversary, but he is not the disguised visitor posing as Týr.