Thanks for the ideas everyone. I think Unseen Seer looks good.
I don't plan on having this NPC catch up to or overtake the PCs in level - she might get a bit closer tho (4th vs 14th is a big gap).
She's mainly meant to be a source of plot hooks for the PCs, so doesn't need to come anywhere near their level of ability - but she should be able to get herself into trouble and need rescuing. (Whether that's getting bailed out of the city jail or saved from the BBEG's clutches or something in between, that's all yet to be decided).