Dump 'em both!
Take Viconia from the Government district (although she's also a romance NPC!) and also grab Keldorn. With these two - Keldorn, with some niffy weapons later on, you can use him as a rampaging tank.
But if you can't then I'd say dump the Jaheria. Sorry, but I'd rather have Aerie with a Robe Of Vecna and have her standing behind me, than a whiny druid. Depends on whether you have a good fighter or not really
- that's why I suggested the Keldorn thingy.
One more note: If you have Jan, don't have Aerie. (these two are first class dying NPCs for their crap HP, you'll need Jaheria's good fighting HP
)
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