It's still debated.
From an AU:
"It's very obvious Ewan and Walker aren't the same person. They have different names for a reason. If they were the same person, they would keep the same own name, it wouldn't be changed later on."
Yet, from Mattrio we have THIS:
"Yes, I can confirm you that Agent Walker and Ewan are the same person. String used in the game for NPC_EWAN always redirects to Walker. It was probably a change made during the development"
I'm going to let you all decide on this one.