It's the other way around.
On one side, there are the canon characters, and on the other, the original characters.
Canon characters are the main antagonists. They won't be part of Ryûko's harem, and most of their "kinky" content will happen during their fight with the protagonist. They are the characters you can see in the red section of the diary.
Original characters are the ones who can be involved in a romance with Ryuko and be part of her harem. They are the characters you can see in the white section of the diary.
There are a few exceptions, with Mako being one of them.
However, I know that a lot of people want to see more scenes between canon characters, so I will include side content for them, but people should keep in mind that most of them will remain antagonists throughout the game.
Feel free to ask me more if you think it's not clear enough.