Just sent some ideas to Zeth, I'll go ahead and post them here too.
I think this is the book that it came from, but I'm not entirely certain:
Witch Book. And yes, she is going to be a human and a woman. I was also thinking about how we could work her in, and came up with a few ideas:
The new character's reason(s) for joining:
1.
The witch from before is some kind of enemy or rival. I'm not sure if the witch we were dealing with originally is still the main antagonist, but if she is then my new character, who I'll just call Alice as a placeholder, might dislike her or be sent to deal with her by the coven she's in.
2.
She owes the party something. Alice could know one of the other people in the party and owe them a favor, which then causes her to come along with them. What she needed from them would vary from person to person, but if it is Jack then she could owe him money or her life after he saved her one time.
3.
A crush on a party member. Pretty simple and could lead to a few shenanigans depending on who it is. Light theme could be a simple crush that leads to comedic moments, dark theme would probably have to be a yandere.
Reason's for Belle's Departure:
1.
Belle is killed or otherwise permanently removed from the story. I'd prefer if this wasn't the method, since I generally don't like to see my characters die. It would be a quick and easy, if a little messy, way to put Alice in, however.
2.
Belle is turned into a cat/rat/bat/other kind of small animal. This probably wouldn't be Alice's doing, but could be some kind of rival or enemy witch or even just an accident. Then Alice comes in, makes her a familiar, and Belle is delegated to the role of an NPC/Hireling. It doesn't remove her, but it will make her take a more passive role in the story. Plus, she could get a humanoid form after becoming a familiar, giving us a catgirl of some kind!
3.
Something happens in Belle's hometown that requires her to go alone. This would be a little anticlimactic, and a bit of a bitter way for Belle to go depending on the nature of what's going on in the hometown. But, it could also leave a future plot point of some kind as a new villain makes their debut.