Cúchulainn is a 17-year-old warrior of exceptional talent and the main defender of Ulster. Often referred to by his childhood name, Sétanta, he takes on the name "Hound of Culann" after swearing to protect a smith's household. He possesses a supernatural combat state known as the "warp-spasm" and fights with a lethal weapon called the gae bolga. Driven by honor and community ties, he attempts to hold off the invading Irish armies alone while the men of Ulster are incapacitated by a curse.

Conchobor is the formidable king of Ulster, born with both royal and mystical blood. He acquires the throne as a young boy and earns the love of his people through generosity and smart resource distribution. As a ruler, his pride occasionally creates deep conflicts, leading to the exile of several key Ulster warriors, but he remains a revered leader who commands fierce loyalty from his remaining subjects.

Medb is the haughty and warlike queen of Connacht. Driven by a desire for absolute parity in her marriage to Ailill, she organizes a massive military campaign to acquire the Brown Bull of Cuailnge. She is intensely proud, dismissive of warnings from seers, and frequently employs sexual manipulation, deceit, and false truces to achieve her military goals.

Fergus is the exiled former king of Ulster, now living in Connacht and allied with Medb and Ailill. Despite marching with the invading Irish forces, he maintains a fierce, secretive loyalty to his homeland and his foster son, Cúchulainn. He acts as an intermediary, guide, and occasional saboteur within Medb's ranks, walking a difficult line between his current alliances and his deep-rooted kinship with the Ulstermen.

Ailill is the king of Connacht who joins his wife on her military campaign to Ulster. He offers a calm, strategic alternative to Medb's reactive leadership style. He values practicality over pride, seeking to avoid unnecessary casualties among his troops and attempting to establish honorable truces, though his efforts are frequently undone by Medb's deception.

The Morrígan is a shapeshifting goddess of war who interferes directly in human affairs. She takes various forms, including a young woman, an eel, a wolf, and a red heifer, injecting unpredictability into the mortal conflicts. She operates on her own motivations, alternately offering aid to warriors or undermining them in combat when she feels slighted.

Cathbad is a druid and seer with strong ties to the Ulster court. He possesses the gift of foresight and arrives at critical moments to deliver prophecies regarding birth, greatness, and early death. His predictions consistently connect the mortal characters to the pre-Christian divine, cementing their destinies in the broader mythology of the land.

Laeg is Cúchulainn's devoted charioteer and closest battlefield companion. He equips his master for combat, manages the chariot during intense single combat, and provides crucial emotional support. He actively encourages Cúchulainn through well-timed insults when the warrior's energy lags and acts as a grounding confidant during moments of grief.

Ferdia is a warrior perfectly equal in skill to Cúchulainn, having trained alongside him under the warrior-prophetess Scáthach. He is proud and susceptible to manipulation regarding his honor. His deep bond with Cúchulainn represents the tragedy of divided loyalties, as the two men find themselves forced to fight one another over political obligations.

Emer is the daughter of Forgall Monach and the wife of Cúchulainn. Before agreeing to marry him, she tests his capability by assigning him a series of seemingly impossible physical and martial tasks. She supports the warrior through his trials, though she protests the strict, deadly rules he imposes on their extended family.

Scáthach is a fierce warrior-prophetess who trains young men in advanced martial arts. She teaches Cúchulainn supernatural feats of combat and predicts his glorious but short-lived future. She commands great respect and oversees the formation of many of the era's greatest fighters.

Finnabair is the daughter of Ailill and Medb. Her parents frequently use her as a bargaining chip, promising her hand in marriage to any warrior willing to face Cúchulainn in single combat. Her lack of agency in these political machinations traps her in the center of a deadly and deceitful game of war.

Deichtine is Conchobor's sister and the mother of Cúchulainn. Her pregnancy involves supernatural elements, including swallowing a mystical creature and receiving dream visions. She ultimately raises her exceptional son alongside her husband, Sualdam, attempting to manage the boy's unpredictable early behavior.

Sualdam is the earthly father of Cúchulainn and the husband of Deichtine. Because he is not originally an Ulsterman, he avoids the debilitating curse that strikes the rest of the kingdom's men. He attempts to assist his son in warning the kingdom about the approaching Connacht armies.

Lug is a supernatural figure from the síde (the fairy mound) and the otherworldly father of Cúchulainn. He possesses deep mystical powers and intervenes directly during the war to place his exhausted son into a three-day healing sleep, singing to him and treating his wounds when no mortal can help.

Nes is a queen who manipulates the line of succession to secure the kingship for her son. By agreeing to marry King Fergus only if he grants her son the throne for a single year, she demonstrates sharp political cunning that alters the power structure of Ulster permanently.

Derdriu is a tragically beautiful woman kept hidden by King Conchobor, who intends to marry her. She rebels against this forced confinement by binding a young warrior named Noisiu to her, prompting an exile that fractures Ulster's warrior class and creates deep-seated grudges among its leaders.

Noisiu is the son of Uisliu and a skilled warrior and singer. When Derdriu binds him to her, he is forced to flee Ulster with his brothers to escape King Conchobor's wrath, setting off a chain of events that leads to exile and war.

Macha is a mysterious, supernatural woman who brings prosperity to a farmer's household. When forced by the king to race his chariots while heavily pregnant, she wins but uses her dying breath to place a devastating curse on the men of Ulster, causing them to suffer labor pangs during times of great need.

Aife is renowned as one of the hardest women warriors in the world. Defeated by a young Cúchulainn through trickery, she submits to his terms, which include bearing him a child and raising the boy under strict, isolating martial rules.

Connla is the exceptionally skilled son of Cúchulainn and Aife. Raised under a strict set of rules that forbid him from yielding, making way, or identifying himself, his arrival in Ulster sets him on a direct, unavoidable collision course with the kingdom's greatest defenders.

Mac Roth serves as the primary scout and messenger for the Connacht army. He provides Medb and Ailill with critical intelligence, surveilling the land ahead and describing the formidable, terrifying approach of the recovered Ulster warriors.