[Okashira]

Chapter 7.... Whoa, so probably now.....

Spoiler:

So, the stalker is either leading him into a trap, or he is going to try to use him to get rid of the imouto (which actually is the girl who gets the most attention out of the MC)

If we think about the little "plot hole" that the guy got the letter at the start of the arc, before the MC started to doubt him, that can be easily solved with "the stalker was also stalking the imouto since long before the arc started", heh