Hi I am starting to read it, but if it seems linear so far... is that right?
It isn't very much likely, because there weren't any reasons to have a choice in the story right now. We have 4 romanceable options, so I'm sure we will have options to choose, not to mention that the story feels like its genre is mistery or thriller and these genres has a high potential for a non-linear story. So don't worry about it, it's not linear in my opinion, it's just the first build anyways. :D
If you're asking if there will be a branching narrative for all the friend, then yes if this follows the story. If you're asking if there will be choices, then yes again if this follows the story. I don't know if the VN will expand upon them, but I think there is about three to five choices once you're locked into a character. The choices leading to a "good" or "bad" end depending. Again, this is all speculation based off the story that might be getting revised or changed for the VN.