Felassan has an unusually sarcastic and playful demeanor. He seems to know a great deal about elven history, and particularly about the enigmatic trickster god Fen'Harel. His attitude is also much more carefree than the grave manner expected of a wise elder, or apostate mage. Though he presents himself as Dalish, no Dalish clan willing to speak with outsiders admits to having had Felassan among its ranks, and they noted that such a gifted mage would more likely be serving as Keeper for a clan than wandering the world alone. Both Empress Celene and Grand Duke Gaspard, who travelled with him for a time, said that he was unlike any City or Dalish elf they had ever met. 
Felassan befriended Briala while the elven woman was making her way to the Dales, and Dalish elves, following her parents' murder. He killed several human bandits who would have attacked Briala. Felassan then took her under his wing as a protégé of sorts, becoming her hahren, or revered elder and teacher. In particular, the mage appeared to teach her about the cunning ways of Fen'Harel, and how to apply his stories to her own life and struggles. He then convinced Briala to return to future-Empress Celene I to continue her goal of championing the elven cause in Orlais.
Later on, Felassan meets with Briala once a month in the alienage of Val Royeaux to share information useful to Briala's position as Celene's spymaster, and sometimes to offer help and advice. He assists Briala in locating Ser Michel de Chevin after his capture by the bard Melcendre, and later passes on to her word of the rebellion in Halamshiral following the brutal murder of the elven trader Lemet.
Felassan travels with Briala to Halamshiral in a last-ditch effort to prevent political catastrophe for the Empress, and assists Briala and Thren in making their way to the manor of Lord Mainserai—the nobleman responsible for Lemet's death--in order to kill him, win justice and end the uprising. However, when Michel and others of Celene's army move in to arrest Briala and Thren following Celene's hand being forced into crushing the elven rebellion, Felassan disappears into the forest surrounding the city to wait for his protégé to return.
The two, lacking any better options, make their way to Dalish clan Virnehn to lose themselves and hide for a time and regroup, ultimately accompanied by Michel and Celene. Ser Michel discovers the clan has trapped the "choice spirit" Imshael, who promises the party the power of the Eluvians if they bargain with him. The eluvians can allow Celene to return quickly to Val Royeaux, and so Briala approaches Felassan to help create a distraction so Imshael can be approached and dealt with while the clan's attention is elsewhere. Felassan's price for this aid, however, is that he will never reveal to Briala whether he is truly Dalish or not, despite his vallaslin.
Felassan and Briala assist Michel in killing alerted Dalish elves to get to Imshael and confront the spirit. Felassan greets him familiarly and the spirit responds in kind, calling him "Slow Arrow": the meaning of his name in Elvish. The party takes a keystone gem the spirit creates to activate the eluvians and Michel unwittingly frees Imshael to murder the remaining members of clan Virnehn.
Felassan then leads the party through an underground chamber where the nearest eluvian lies, and then into the magical pathway between the portals. As the party is joined by Grand Duke Gaspard de Chalons and his own companions, Felassan displays unusual knowledge of the elven burial chambers, magic and corpses--both revenants and "demon-mages", as well as the corpses of elves who had been dreamer mages in Uthenera--that the party sees and combats on the way to a main chamber where Imshael's gem can activate the sleeping eluvians. He speaks with bitterness on the cruelty and decadence of Elvhenan, noting that for all its magical glory it was no better to its servant class than modern-day Orlais is to its city elves.
Upon reaching the main chamber, Felassan assists Briala and company in defeating a varterral guarding the activation pedestal and elven rune labyrinth. He also appears to silently support Gaspard's elven comrade, the clan Virnehn survivor and First Mihris, when she opposes Celene's attempt to stop Briala taking control of the eluvians with the gem following a duel to decide the future of Orlesian rulership. When Mihris reveals she is possessed by Imshael, Felassan persuades Imshael to leave her, with the enigmatic promise of "something bigger" in return.
After Briala transports her companions away to various locations throughout Orlais to continue their own struggles, both personal and political, she offers to give Felassan the passphrase she created to utilize the eluvians. He prevents her from telling him, however, telling her he must go and that he has more work to do. When Mihris asks him if he will take her to his clan, Felassan refuses and informs her that she would not want to meet his clan. He then kisses Briala on the forehead in farewell and disappears.Alone, Felassan enters the somniari trance to speak with a mysterious entity in the Fade. With a sense of prescient doom, he informs the entity that he did not get the passphrase from Briala, and suggests that she be given a chance to try using the eluvians. He remarks that "they" are stronger than the being thinks and attempts to tell the entity that Briala reminds him of something, or someone, else, but is killed before he can do so. His last thoughts are of Briala, alone and in search of her people.
- According to Patrick Weekes, Felassan likely bears the vallaslin of Mythal. Beyond that, he would be a very "elfy elf" in appearance, not unlike Solas, Ameridan or Abelas.