Hello there. Thank you for visiting this thread and for leaving your question. I won't divulge into much as I wish to limit what I reveal; however, I can confirm that yes in a
nutshell story short there is
no harem in this game. I am not saying that there is not a possibility to "try" and pursue more than one girl at once (within limitations and based on how I write and decide to direct the story); however, be warned that doing such a thing if and where I allow it as an option can result to deadly consequences. This also takes into account that there will be deadly ones regardless... Also, take note that if I had picked to make this game a harem, it would not only drag more screen time trying to balance out the "harem" (which many do struggle to do) but it would shatter the pacing and plans for this story, which ties into my point below.
Remember this is a horror slasher game first and foremost.
As for the later part of your question the highly likely outcome would be that "if" you've focused your energy in raising RP points (relationship points) with a certain character and focused on them, then yes you'd be relationship wise eventually locked in on them.