itch.io

you can get in a relationship with the manticore if you get addicted to her venom after talking to her a bunch(just keep taking the h scene as far as she allows and then go right back to her, around 10ish times)

Addition to what the other user said: if I remember correctly, you'll also need to win a few times against the Manticore while in vaginal sex stance, in order to get in a relationship with her.

As others said, you can get into a relationship with her
You can also know her name and go into her den

You also can take her spikes and inject it into yourself to increase your addiction.