iirc, you spot her and tell her to go away with low charisma, then you have the option to bribe or fight.
Usually she's available early on, if you don't find her, the new region at the top has more, but she shouldn't be that hard to find, just upgrade your scouting perks and you'll see her, I think she's
1)forest
2)human
3)brigand
I usually have at least one by the time I reach the river where the mermaid is.