Most don't have several routes but Samantha for instance has a few different approaches on how she ends up.
At first the route is pretty straight forward with you going to her and Ryan's wedding. Then after the wedding and depending on your choice during the wedding one of several routes can occur. You could also have caught Ryan cheating and told Samantha which will stop the wedding.
Personally I think there are 2 "main" paths to go with Samantha, either A) she leaves Ryan (how it happens can be done in multiple way's) or B) She stay's with Ryan but is either unhappy or sort of happy with him.
Now how you reach either A) or B) is up to you and depending on how you do it, it will have different outcomes on her max KP and LP but also if she is open to be in a harem or wants you to be exclusive with her.
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No, see that's the problem. I have done that, following the in-game walkthrough
I wait until Saturday. I go to the arcade. 9am, 10am, 11am...all the way up till it closes. Nothing
Make sure to check the settings if you have randomness enabled or not. The wiki assumes that randomness is turned of since then the girls all have set schedules on where they will appear and at what day/time. But if it's turned on then well it's all random and you just have to hope that Bree turns up at the arcade.