Appearance
Emerald Hues is a female unicorn with a mint-green coat, pale green mane and tail, and blue eyes. She has a hexagonal cut emerald sparking with magic as a cutie mark. She represents the element of magic in her time.
Personality
She is highly intelligent and has been extremely talented in pursuit of academic knowledge about and the practice of magic all her life. Unlike her far-distant counterpart Twilight Sparkle, however, she's done so without the benefit of having been spotted at a young age and placed under special instruction by a tutor of similar or superior ability. As a result of being more intellectually skilled and often able to outwit, out-maneuver, and out-manipulate other ponies and creatures around her, even those in ostensibly higher positions of authority and privilege than herself, she tends to have little regard for respecting limits, barriers, or being told no, seeing these as puzzles and challenges to circumvent or overcome to get what she wants rather than boundaries to accept being limited by.
An example of her mentality would run approximately as follows: If someone locks the door, go in through the window. If they lock the window, break the glass and go in anyway. If they put bars over the window, telekinetically grab a log and battering ram the locked door until it breaks open. If all else finally fails, just set the place on fire, whoever's inside will be forced out and they'll probably bring whatever was of value that you wanted access to in the first place out with them, and then it's just a simple matter of using the commotion and confusion of the fire to seize your opportunity to grab it and run. Every problem has a solution, it's just a matter of figuring out the puzzle.
Despite this attitude, she's not a bad pony or sociopathological. She's just not particularly socialized, largely by her own choice due to not having seen a great deal of value in becoming so - possibly analogous to Twilight Sparkle if she'd been allowed free reign to continue on her own to grow intellectually and in magical ability but was never assigned to Ponyville to learn about finding and valuing friends. She isn't nice simply for the sake of being nice or friendly or to make others feel better about themselves. She aims for her own interests to be fulfilled as efficiently as possible and equitable positive exchanges with others don't mean much to her except in the context of when they're the best and easiest way to achieve what she's after. As a result, when she doesn't believe that being nice is the most direct path to own ends, her interactions with other ponies (and sentient life in general) can often be blunt, rough, or crude, involving sarcasm, misdirection by omission, or open contempt for those she thinks are significantly beneath her own brainy gifts of insight and problem-solving. With those who are capable of approaching her own level, she utilizes evasiveness, careful observation and fishing for clues and windows into their personality and potential exploitable weaknesses, and playing her own cards close to her chest to prevent her own games and her true pursuits from being unduly revealed to someone with an interest in impeding them. Not being very social and not caring much about opinions held by others, she has relatively little concern for being embarrassed or humiliated and will sometimes be willing to inflict these on herself if it allows her to drag someone else down with her in a way that accomplishes something she's after.
(To put it all more simply: On the most basic level, this character's personality is inspired by the TV character of Dr. House. If you're familiar with that show, you pretty much know who Emerald is too!)
Background
She descends from a long line of ancestry said to be of pure-bred unicorn pedigree, without any earth pony or pegasus contribution of parentage. This is certainly true at least within living memory, she has no genetic relatives that lack a horn, and most of them take pride in this fact (although the occasional differently-minded cousin does sometimes decide to date an earth pony or pegasus, and this is usually tolerated without any negative stigma or rejection but with the understanding that any offspring from such a relationship will not be recorded in the family tree as a "pure" unicorn). Some of them go so far as to attribute her considerable magical talent to this tradition of preserving and cultivating a concentrated unicorn-only bloodline. There are even rumors, encouraged by the most proud of them, that the speculated ancient alicorn Princess Twilight Sparkle was born a unicorn and related to a primordial scion of this family tree. Emerald, for her own part, doubts this claim, due to a combination of the lack of any real empirical evidence able to prove the ancestry of any pony so far into the past and doubts about the historicity of "princess" Twilight Sparkle in the first place. A princess alicorn said to live, of all places, in the local library of a small town seems too implausible, compared with the known high quality of living conditions enjoyed by the two historically verifiable princesses, Celestia and Luna.
Emerald has not dedicated herself to a single professional specialization, rather she enjoys working with magic in general and has had a wide variety of employment in fields involving using it, although getting bored easily in some tasks can become a problem. She takes opportunities to travel when they're available, in a continuous search for a combination of the most advanced magical and technological knowledge of her day, what she can derive and learn from the old Equestrian remnants of far more advanced magic and technology, and the adventures that give her the occasions to put these skills to the test in real-world use. She has a particular affinity for prospecting valuable gemstones via magical means, which in her youth earned her her cutie mark (although she's since come to feel that using her magical skills for gemological purposes was the circumstance of the time simply by chance, and that her mark is more rightfully representative of the passion for magic in general than for a single application of it).