Undines are probably the most flexible of the planar-influenced humanoids. (I want to say “planetouched,” but I suppose that’s strictly a 3.0/3.5 term.)
For one thing, the Plane of Water simply has more creatures that can plausibly comingle with humanoids, adding tritons, nereids, and others to the usual list of genie and mephit(?) bloodlines. (Those must be some particularly charismatic mephits. Or maybe mephits doulas have a habit of blessing water births.)
Also, in folktales the spirits of rivers and seas are always having children—it seems like every other Greek myth or Miyazaki film has a river god for a dad—and the undine stat bock is a great way to represent that. (Ditto if you want to replicate the Forgotten Realms’ spirit folk on the fly: undines fit the bill with no conversion necessary.)
And finally, as with the other planar-influenced humanoids, the Advanced Race Guide is pretty much an absolute must for undine feats, spells, variant abilities, class options and archetypes, etc. An undine bard with powers reminiscent of her nereid mother or an alchemist with ooze breath is going to have quiet a different feel from the default cleric in the Bestiary 2…
The villagers of Camden Hill have always lived just up the slope from “the River Folk” in Camden Bottom…and have always considered them just a bit different. That’s because they are—the River Folk are all undines, though none of them know the term (“River Folk” has always suited these human-seeming fishers and traders just fine). Like all traveling people, the River Folk tend to be welcomed and made scapegoats at the same time. Recently the suspicion has been well earned—Fintar Brilson offended some freshwater scrags, who then tracked him back to the Bottom and caused considerable damage. Meanwhile, Madelina the Younger promised her firstborn to a sea hag to cover a debt, and now that she is in her 40th week the witch is bound to come calling.
An undine cleric, the chief water priestess of Sky Harbor has always been a force no right-thinking captain would cross. But recently her pronouncements have gone from being stern but good advice to harsh and erratic edicts. When she closes the harbor and sends water elementals patrolling the docks, the captains are outraged. Has she gone mad? Is she in the thrall of some power? Or does she know something they don’t?
Merisal was an alchemist fascinated with the elemental power of water. Eventually her experiments with mutagens and mephit essences were enough to turn her into a kind of undine. But the metamorphosis is not a stable one, and she needs rare and increasingly contraband components to maintain her form before she turns into an oozy horror.
—Pathfinder Bestiary 2 275
Wikipedia tells me that undines are alchemical spirits, so naturally I had to add an alchemist adventure hook.
By the way, insert my usual grumble about the use of the undine name (though my case is weak compared to oreads and sylphs; undines have never been that strongly defined in myth or role-playing).
And stay warm, all of you! Unlike undines, I’m guessing you don’t have resist cold 5…