Jill will leave after you eat someone 4 times. You can get her to stay 5 times if you pick the right dialogue option, but other than that there's no other way to keep her around longer. You can recruit Nessie on the vore route after defeating her in her cave. Or recruit her in the normal route during The Depths sections.
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I defeated nessie on normal route is depths section later than that ? Also ate there any plans to make a vore route easy mode where you can devour any npc and gain permanent stat buffs or new abilities that get stronger the more you devour? and is jill leaving what determines whether your on the vore route?
The Depths section is much later in the game. As for the vore route, you do get stat buffs and new skills when you devour certain characters. It's meant to be harder though. Jill leaving is a sign of starting the vore route, but it can also be started with our her. When the Devourer gives you the Devour skill is when it starts.