"Be careful what you wish for. I suspect your intellect to be ranked somewhere between the Spiderman musical and buying a solar powered flashlight."=====
Stella is a mysterious winged unicorn who only appears once every year, when the Scarlet Shooter passes over Equestria.
Not much is known about Stella, as she can only walk among mortal ponies once per year, when the magical "Scarlet Shooter" star comes into Equestria's orbit. The mythology is that she will grant the wishes of various ponies, but out of boredom or malice will always demand something precious in return.
A more benevolent myth suggests that Stella is the Equestrian equivalent of a genie, once being a mortal pony and having her current state be the price for a wish she once made. In this myth, the Scarlet Shooter is a curse started many centuries ago, possibly with/by someone in Celestia and Luna's family.
Stella has a trickster's sense of humor, toying with the fears and wishes of ponies for fun. Mostly she justifies this with her boredom from hardly ever interacting with others. Stella is behaves flirtatious and carefree, claiming to look down on friendship, though the nature of her existence is implied to make her quite lonely.
While she's not exactly an antagonist, she does cause a bit of trouble every time she visits Equestria. Even though she is passionate about the need for dreaming, she is not above taunting ponies with their fears or tricking them into making a wish. Her motif seems to be that of dreams rather than wishes- claiming that nothing worth having ever comes easily, but that one should always dream. It's often wondered if Stella likes her job at all, as she considers wishing one's dreams into fruition to be "cheap", and that any pony that does so deserves "what they've got coming to them." This reinforces the theory that Stella was once a mortal who made one such wish, and resents herself for it. Though eternally paying the price for her choice, she seems noble enough not to trick another pony into taking her place.
Stella resembles an adolescent winged unicorn with a glittering black mane, white fur and red points. She wears ruby cuffs on her upper legs, and a small silver crown. It's been wondered if she's somehow related to Luna and Celestia, but Stella merely shrugs when asked- implying either that the origins of the Shooter are a mystery to even her, or that she is and just doesn't think it matters. Her cutie mark represents the Scarlet Shooter.
Along with flight, Stella has the power to know a pony's dreams and desires, as well as their fears. She can grant wishes, but afterwards the magic will come full circle and bring the pony bad luck for equivalent exchange. She also uses invisibility to sneak up on or spy on ponies.
Stella is apparently immortal, and implied to somehow be the Scarlet Shooter, in turn suggesting that her life force is tied to the star/comet. When the Shooter leaves Equestria's orbit (after the span of a week), Stella has no choice but to leave with it to places unknown. Stella also has some illusionist ability, sometimes embellishing and showing ponies their own desires or fears.