<Spoiler but I recommend you read anyway> You CAN ROMANCE KLAUS!!!!! ok so actually wair hang on. First of all im not sure if this is truw yet, BUT i just discovered an option in the game I didn't before and i think you probably didn't either! Because it's still on day one, when you're still on the way to devons route. After asking him for help w the key and asking to share the room w him, you know how he has to first put away the telescopes, yeah? Well if you choose to not help him w the telescope and just go inside you can encounter klaus on the cafeteria, and choose to follow him! I just came from a long reddit thread w someone else who knew this and they said this is thw way for klaus route. Idk how true this is bc I literally just got therw but will reply to this comment later whether or not its possible!
that's not the case, nope :(. it's been confirmed by the author that Klaus' route is "on hold" until they're sure that they're gonna do a good job with it. rn, D2's end is where the route stops and there's no content beyond that (even if the lack of a credits scene would imply otherwise). soooooo unfortunately, we'll have to wait for more Klaus content, as rn there's no way to open that door. but his route will be expanded on the future! ;)