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Yes.

Step 1: Training Evaluation - Train her yourself, do not share her with Don.

Step 2: Do not share the one on the beach.

Step 3: Do not show Lilly the photo.

Step 4: Accept the offer from the Boss (Dark route).

Step 5: For the Nani choice, do not select the third option. 

Step 6: If Sarah is still in the mansion being trained, you have the option to switch her to the good route to avoid her NTR.

Step 7: Any and every choice that has the label in parentheses (Sharing) or (NTR) - avoid all of those.

You can keep most characters in Dark Route for yourself. A few may not like what the MC becomes, choosing instead to walk away.