The chosen character becomes the elder sibling in the game's narrative, who is stranded from childhood on the island of Kephalonia following a traumatic incident at Mount Taygetos, and as an adult becomes a legendary mercenary known as the "Eagle Bearer".

Kassandra in particular has received attention as a positive example of gender equality and representation in video games, as well as critical acclaim in response to Mahut's performance and interpretation of the character.

Whoever is chosen by the player as Layla in Odyssey is revealed to be the legendary mercenary known as the Eagle Bearer who was active during the Peloponnesian War, and whose exploits are detailed in the purportedly lost accounts of a version of Herodotos in the Assassin's Creed universe.

For the game's novelization written by Gordon Doherty, Kassandra appears as the Eagle Bearer who opposes the Cult of Kosmos and eventually inherits the Staff of Hermes Trismegistus from Pythagoras.

His allegiance to the Cult puts him at odds with his sister, who attempts to dismantle their influence across the Greek world in the midst of the Peloponnesian War and to reunite their family, and later dies fighting Kassandra at Mount Taygethos.

In an interview by Tom Hoggins from The Telegraph, Odyssey senior producer Marc-Alexis Côté said that the Kephallonia section early in the game is "about discovering who these characters are and find out why you should join them on this journey".

[11] At the beginning of the process, the creative directors and writers of Odyssey gave both actors a "skeleton" idea of their characters as well as some insight as to their history, possible path and general characteristics.

Antonakos and Mahut made it a point to work as a team, often holding discussions with each other about where they saw their respective characters going, and what they thought their main attributes were, and how they would respond in key situations.

[11] On the character's display of machismo, Antonakos said Alexios is essentially a demigod with supernatural strength who is also immersed in the Ancient Greek warrior culture and explained that his arrogance is "just a show, a mask for his insecurity of being alone", noting that he is in truth "a big kid acting tough" with "huge guilt issues" and "something to prove".

In 431 BC, at the onset of the Peloponnesian War, Kassandra uncovers the existence of a secret society called the Cult of Kosmos when she is hired by one of its members to assassinate a Spartan general known as "The Wolf of Sparta."

After befriending a naval captain named Barnabas and assuming command of his ship, the Adrestia, Kassandra travels to Megaris to fulfill his contract, only to discover that "The Wolf" is Nikolaos and that the Cult is targeting her family.

During her journey, Kassandra also collects various Isu artifacts at the request of her biological father Pythagoras, the assigned Keeper of the Staff of Hermes Trismegistus and guardian of the secrets of Atlantis.

The second story expansion, The Fate of Atlantis, is set after the events of the base game and follows Kassandra's quest to learn how to unlock the full power of the Staff of Hermes after it was passed down to her.

[12] She is featured in the questline titled "A Fated Encounter", where she comes face-to-face with Eivor Varinsdottir, the main protagonist of Valhalla, whilst hunting for an Isu artifact that is causing nightmares amongst the locals on the Isle of Skye, during the late 9th century.

The DLC establishes that, ever since the deaths of all of her companions, Kassandra has become very solitary and anti-social, refusing to stay in a single place for too long or form connections with people due to her knowledge that she would eventually outlive them.

Her story arc is set in 404 BC, nearly two decades after the conclusion of Odyssey, and follows Kassandra as she returns to Greece to deal with the Spartan oligarchy installed in Athens known as the Thirty Tyrants[14] Although Kassandra is designated as the canon Eagle Bearer for the novelization and was featured prominently in gameplay demo footage shown at E3 2018, the vast majority of the marketing campaign and available merchandise for Odyssey exclusively featured Alexios, including the game's three pre-order statues, cover art, official trailer, header image on Uplay, and so on.

GLAAD credited the positive depiction of Kassandra and Alexios as sexually fluid characters to be one of the rationales behind the nomination, though the controversy generated by the Legacy of the First Blade DLC pack was also acknowledged.

[18] Xalavier Nelson Jr. from Rock, Paper, Shotgun was of the view that both are "distinct, compelling, and valid protagonists" in their own right, and that the subtle variations between both choices colours the events of their game moreso than the differences between a male and female Commander Shepard for the original Mass Effect trilogy.

[19] On the other hand, both Ali Jones from PCGamesN and Fraser Brown from Rock, Paper, Shotgun questioned the Alexios-centric marketing campaign for Odyssey and the lack of promotion for his female counterpart.

[15][6] Joe Parlock opined that the executive-mandated decision to have an interchangeable lead made it more difficult for the writers to develop either Kassandra or Alexios as their own characters in response to the Bloomberg report by Schreier.

[31] Samuel Axon from Ars Technica said Kassandra stole the show for him, and that choosing her for his first playthrough felt like a fresh experience; he liked the notion of a female character with agency roaming ancient Greece "liberally partaking in all the violent and sensual pleasures on offer with Conan-like revelry and impunity", framing an entirely different tone and context not unlike that of Xena: Warrior Princess.

[22] Writing for PC Gamer, Fraser Brown formed the view that Kassandra is "immediately likeable" due to her "easy-going, roguish charm", and compared her "longstanding popularity" to that of Geralt of Rivia from The Witcher media franchise which end up overshadowing the Assassin's Creed series' other protagonists.

[33] Colin Campbell from Polygon praised Kassandra as "one of the most complete human characters" he has ever played as in a game, noting that she comes across as "smarter and funnier" then her male counterpart, and that "her facial animations are a joy to watch, while her vocal reactions generally match her movements".

Noelle Adams from Critical Hit praised Kassandra as a physically strong and tough survivor who combines likeable traits from many other female protagonists in fiction and yet has a malleable personality, in addition to being better animated then her male counterpart.

[27][28] Kelly considered Alexios to be well acted, but felt that Antonakos played him "far too straight", which in his opinion failed to differentiate the character from "a dozen other gruff videogame heroes".