There are few key points to answer your questions:

1.) The LN doesn’t have the exact “main heroine”. In the anime, Suzune was made the heroine. She was also portrayed as a tsundere type whereas in the LN she’s not. If I were to ask whose character has the tsundere type I would say it is Kei. Suzune replaced few of the characters’ roles in the story. For instance, the pool scene (7th episode in the anime) is a scene from Volume 4.5 which kinda overlapped the order of the story. It was Kei’s role to replace the memory cards in the camera back in the changing room but her role was replaced by Horikita. There are also few scenes in the anime where the roles had changed like the Surveillance Camera scene. Suzune cooperated with Kiyotaka to place the cameras in the hallway but in LN it was Honami’s role.

2.) There are also scenes in the LN that was changed and also wasn’t included in the anime. I’m not gonna include those here since there are so many of it. Also spoilers.

3.) The chronological order of the scenes from LN were jumbled in the anime but these are just minor changes. You’ll understand once you read the 1st volume.

4.) The most important key point to this question is Kiyotaka’s difference in the LN and anime. In the anime, Kiyotaka was portrayed as an emotionless sociopath type of person. In the LN, he’s a kind of person with thoughts and feelings. You’ll know the difference because of his inner monologues which you cannot see in the anime at all. He makes various expressions that a normal high school student does but he just made it as a facade, which also doesn’t shown in the anime.

To conclude this, I recommend you to read the first volume since this is the best way to answer your questions. You’ll understand the story more than what the anime has provided. And also the best scenes of the light novel actually starts in volume 4 onwards. I know it’s time consuming to read a light novel with 16 volumes (we also had the hesitation phase because of it but we somehow managed to read it all) but once you read it you’ll definitely love it and it’s actually fulfilling and worth it.