SPOILER
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I think the Walter the others knew is actually Herbert, judging from the reaction of "Herbert" in one of the dreams, (like "I'm telling mother"), I think they did a switcheroo. I just hope it isn't the same situation as Mark
I think Herbert actually died and Walter is just delusional about having a brother in gils route he mentions that Herbert has been different since the accident like how mark also said that his brother was different ever since. And maybe Walter has a split personality disorder that would explain the situation with Mark being stalked by someone who looks like Walter. But yeah that's all just a theory