Notes from the Nails: this one is largely inspired by the video game, Baldur's Gate. Though my sirines are more like sirens than water nymphs - wild, beguiling and not bound to a particular place.
A sirine is a beautiful, amphibious fey creature often found on beaches and riverbanks. Humanoids regard them as unpredictable and dangerous, but the reality is more nuanced.
To a sirine, nothing is more important than living in the moment. They act as they please, seeking joy and pleasure wherever it may be found - especially if that happens to be by the seaside! Sirines love the fresh breeze and the feel of warm sand between their toes as much as anyone else; staking out the best spots on beaches to make sure their frolics attract just the right amount of attention.
As with most fey folk, their emotions are strong and shift rapidly, such that they may want to kiss someone one minute, and kill them the next. And a sirine's commitment to spontaneity is deep: they do not hesitate to act on these impulses. This can make them seem wild, but to a sirine, it is the natural state of things.