Agravain, known as Sir Agravain or Agravain of the Iron Hand, serves as a Knight of the Round Table under King Arthur. He is Morgan le Fay's son, brother to Gawain, Gareth, and Gaheris, and half-brother to Mordred. Initially sent by Morgan as an assassin within the Round Table, he ultimately pledges his service willingly to King Arthur, convinced she is Britain's ideal ruler. Agravain acts as the Round Table's secretary and is among the few privy to the king's true gender. His discovery of Queen Guinevere's affair with Sir Lancelot leads him to threaten the queen using the king's secret, setting in motion events that culminate in his death at Lancelot's hands during Guinevere's rescue.

His appearance reflects his title, featuring slicked-back black hair, pale skin, and black armor adorned with a dark blue fur-lined cloak. He often wears a stern expression; during his duel with Lancelot, he appears covered in blood, revealing sharp, fang-like teeth. Agravain is characterized by calm calculation, ruthlessness, and a preference for pragmatic solutions. He consistently argues that the king possesses dragon-like magical power beyond human comprehension and should not be judged by human values. Despite his loyalty to Arthur, he harbors a deep-seated hatred of women, stemming from his mother Morgan's influence, which intensifies upon discovering Artoria's gender during the exposure of Guinevere's affair.

Summoned by the Lion King (Goddess Rhongomyniad) during the "Camelot" Singularity, Agravain chooses to support her "Holy Selection" alongside other Knights forming the Knights of the Lion King. Unlike his peers, he refuses the Lion King's Gift, fearing becoming her Servant would compromise his ability to act against her if necessary. He plays a central administrative role, coordinating the knights, enforcing the Holy Selection by eliminating resistance, and overseeing strategic operations against threats like the False Crusaders and Chaldea's forces.

Agravain's tactical acumen and ruthlessness define his actions in the Sixth Singularity. He reports Gawain's failures to the Lion King, intervenes to prevent Tristan from executing Gawain, and questions Mordred's deployment. He personally oversees the torture of Hassan of Serenity to locate the First Hassan. Confronting Chaldea during this interrogation, he attempts to recruit Xuanzang Sanzang to replace Gareth, debating the Lion King's methods. He later orders Lancelot to eliminate Chaldea's group, showing no concern for refugee casualties, driven by his belief in maintaining order for the king's vision.

In combat, Agravain is formidable despite rarely serving on the front lines, earning the epithet "Agravain who knew no wounds" for returning unscathed from battles. He is infamously skilled in interrogation and torture, described as capable of making "a hippopotamus squeal for mercy in human language." He employs specialized chains known as the "Iron Punishment" or "Iron Mandates" designed to bind Servants and demonstrates the ability to enhance Enforcement Knights into Berserker-like entities. During his final duel with Lancelot, he uses a self-enhancement ability to fight fiercely, managing to return mortally wounded to Camelot to fulfill his duties before dying.

Agravain's complex loyalty to Artoria defines his character arc. While he served Morgan initially, he transitioned to serving Artoria out of genuine belief in her as the rightful king. His loyalty contrasts with Bedivere's; Bedivere admires Artoria's humanity, while Agravain venerates her inhuman perfection as a ruler. This devotion keeps him loyal to the Lion King's ruthless incarnation, whereas Bedivere rebels. In his dying moments, Agravain confesses his true wish was not to serve the king for the kingdom's sake, but to build a perfect kingdom solely for his king—the one person he never wished to be hated by. His death accelerates Camelot's fall, with Bedivere noting Agravain was perhaps the most crucial figure holding the Round Table together.