He is developed from the Author's 'main character' from actual gameplay, and has since been adapted into his own series of fan fictions stories, as well as having been 'borrowed' by several other authors into their own stories.
Sethyas Velas was born and raised in the Imperial Province of Cyrodiil, with a twin sister at a Julianos Temple Orphanage within Cheydinhal County. Here he was trained how to read and write by copying manuscripts for the Monastery, and how to farm and produce wine. These skills would later evolve into a passion for literature and journaling, as well as Alchemy.
Upon reaching the age of majority they were sent out to eke their own living. However racism and Sethyas' odd appearance as a tattooed Dunmer ensured they had minimal work and resources.
In reaction to this, Sethyas began to secretly take up banditry alongside their wanderings. They wandered the whole of Cyrodiil as it was a childhood dream of theirs. His secret trade taught him the use of Bows and Blades, especially Daggers, and how to sneak relatively well.
However, around the age of thirty or so, he was finally caught, imprisoned, and without word or reason, was sent to the eastern Nation of Morrowind, and released into the Service of the Blades...
After acheiving his destiny as the Nerevarine, he learned that he was functionally immortal from his Corprus Disease, in that he would never age or contract disease. He was however still able to die from mortal wounds and injuries. Regardless of this fact, he has shown himself to be extremely trained in even the most harsh of situations, and as a result of these acquired skills, alongside other benefits he has been granted along the way, he is extremely difficult too kill.
Two hundred years later, as his wanderings of the world brought him too Skyrim, he learned that he was, in fact, a 'Dragonborn' as was written in the Prophecies of the Nerevarine; "..but far-star marked, and Dragonborn."
Sethyas Velas is portrayed as a 'typical' Dunmer in the 'grim and aloof' aspect, but an extreme opposite of the 'disdainful of the other races' one. While he does harbor certain resentments against the upper class, and the Empire, he seems to be disdainful only of bigots and slavers, and perhaps this is a complex of a lifetime of alienation in his native country of Cyrodiil. He has been shown to lose his temper with Nobles, Slavers, and people who try to elevate themselves above him in regards to wealth or affluence.
While he rarely lets it go beyond words, he once was shown to have killed a group of Slavers by poisoning their drinks, after overhearing their mocking and degrading speech in relation to Argonians. He in fact seems most comfortable with Argonians in Morrowind, for their 'Kindred Spirit' and calm demeanor. Whether the same could be said of the Argonians around a Dunmer, is left to the readers imagination.
While rarely speaking to people he doesn't know, he has been shown to be an avid conversationalist. Despite having the low, gravelly tone of the Dunmer, he is very well-spoken, a natural narrator, rarely curses or even raises his voice. He seems to enjoy philisophical debates in particular, and will often play 'Devil's Advocate to keep the conversation flowing.
He is extremely well-read, having spent most of his childhood lost in books as he rarely interacted with the other children. As such, he has been shown to be able to keep up with Scholars on many subjects, and has a fascination with ruins and lost cultures.
He is shown to be an efficient and remorseless killer. His first kill was in self-defense, and in defense of his sister. Standing his ground to an Adventurer, he simply stabbed him in the gut with a common belt knife and pulled it upwards. He wrote of this moment as one of awakening, saying that there was nothing in him after he spilled blood, that was not already there before. He has made several entries in his journals to the effect of not being riddled with guilt or visited by the ghosts of his victims. In spite of this trait however, he seems aware that it is not 'normal' and he shows great empathy and compassion to the pain and suffering of his fellow man.
He has been shown to have adopted a strict code of honor since his arrival in Vvardenfell, especially in regards to the slaying of innocents. The Morag Tong mantra of 'Take life seriously, if taking life is your profession.' seems to be a particularly strong tenent for him. He will not kill an innocent, except in all but the most dire of circumstances, and has been known to go to great lengths to avoid this path, or outright protect them. He is especially protective of his adoptive neice, Athynae Sarethi. The only character that he has been shown to be wholly open with in regards to his emotions and thoughts, and has trained her in the arts of war often serving martial lessons with moral ones. However, having lost a wife and sister seems to have developed into a quiet but intense preoccupation with her safety, and has been shown that he would not hesitate to kill anyone who puts her in harm's way.
Ultimately, the people in Sethyas' life have shown great affection and loyalty to him, and he feels indebted to each of his friends, going so far as too say that he does not have friends, only family. Depsite this, he still is known for being very reserved even around his closest friends, which he openly acknowledges in his journals, saying that it was simply his way.
He is also recently revealed to have had an on-again, off-again affair with Serene Catraso Sarethi; the Archmistress of House Redoran, throughout the years. It was revealed through both his, and her journals that around the time the two of them were becoming engaged to their respective spouses an attraction sparked in their meeting. However, after Sethyas' sister died in a confrontation between the siblings, Serene could not turn from the Mer in such pain,and he in that same pain reached out too her for comfort. This occured again not much later after Orvas Dren attacked Sethyas at his Wedding with Ilmeni Dren; the former's neice, and Ilmeni was killed during the attack. In spite of this, Sethyas ruminates heavily upon the deed, and seems to have a great deal of self-recrimination for the affair, which he cannot bring himself to either walk away from, or forgive himself for. The two seem to be intensely in love with one another, and all the conflict and drama that come with it.