At the end of the manga, Sayaka is taken away by Washio Suguru to be a part of his harem, an NTR ending you might call it, he kisses her and takes her away
Now, I’m gonna spoiler things from the second manga Kosuke along with a few members of his resistance group and the new MC Takuma infiltrate the institution in order to rescue Sayaka, when they finally find her, she doesn’t recognize Kosuke other than him being in an island survival, it was then revealed that she was in an experiment that would increase her physical potential in exchange for her memories and emotions
Kosuke was happy to see her but sad that she’s no longer a virgin, then being told that she never had any intercourse, so there was actually no NTR in the end, later on, Kosuke and Sayaka have sex and she was able to snap out of her trance and remember everything
Actually, the first Destiny Lovers is 104 chapters long, the second Destiny Lovers is at 97 chapters and ongoing