You know, people collect a lot of weird shit. This applies to most species, too, as it happens. Well, most sentient, spacefaring species. Some collect worlds. Some collect money. Some collect fingernails, or scales, or whatever. There's an endless variety....
... Navani collects cuties and beauties. Girls, especially. Age doesn't matter, just attractiveness!
... while this may seem shallow, it's no stranger than any other reason for collecting, now is it? She wants to collect them, so she does!
Navani, here, is what happens when you take an ancient, powerful race, and give them good technology without enough to keep them busy. She is, quite simply put, bored to tears with the normal things her race does. Even the novel ability(by our standards) to change her gender at will doesn't keep her entertained! Nor the height-change, nor the various other modifications. However, traveling the galaxy, she has found something to keep her entertained...
Collecting. Specifically, collecting PEOPLE. Things are fine, well, and good, but most of the nice things, while shiny, are ultimately worthless outside of the society that made them. Or, at least, the society that collected them in the first place. Cute girls and boys, though, are MUCH more entertaining in a great many ways, and with the resources of her people, Navani can find a great many ways to be entertained....
Standing(typically, anyway) at a modest 5'9", and weighing an unremarkable 150 pounds, Navani would seem to be an elf, at first. A night elf, perhaps, to judge by her purple skin tone, but that's an illusion easily dispelled. Beneath her clothes, and on her cheeks, she sports a stripy, tiger-ish pattern of blue skin against her primary purple shade, and her eyes, while luminous, are not ALWAYS so. Typically, she's clad in a snug, form-fitting jumpsuit that is surprisingly useful, and protective, but things are variable.
The suit, for example, can retract into a single bracelet. Navani herself can alter her gender at will. She can also travel between worlds and across dimensions, with impressive ease. Her hair, while long, is prehensile, and can do itself up, or be used to scoop up tinies, or tasty prey. And, well, she can be a HE, or somewhere between a he and a she. This serves her purposes nicely, in collecting cuties. Just watch out, lest YOU be collected too! Her appetite for cuties is not just for collecting, after all...