Atla Fayon is a significant character who joins the story as a slave purchased by Naofumi Iwatani, the Shield Hero, alongside her older brother, Fohl. She is a young demi-human of the Hakuko race, appearing to be around ten years old, with pale skin, glossy black and white hair, and ears and a tail similar to her brother's. When first introduced, her body is entirely covered in bandages hiding painful, festering sores, making her appear almost like a mummy. From birth, Atla suffered from a severe hereditary illness that left her blind, unable to walk, and with a very short life expectancy. As her condition worsened and debts from treatment mounted, she and her brother were forced into slavery. While Fohl was compelled to fight in the colosseums of the Zeltoble Consortium, Atla lay suffering, finding comfort in the various languages and stories her brother would teach her upon his return. Her destiny changes when Naofumi, seeking to acquire Fohl as a fighter, purchases both siblings and uses a powerful Yggdrasil medicine, an elixir of miracles, to cure her. After two doses of the medicine, her healing begins, and she is able to walk almost immediately.

Atla possesses a remarkably strong-willed and intelligent personality, often demonstrating a maturity and decisiveness that belies her young age. She has an uncanny ability to cut off anxiety and calmly assess any situation. Her most defining trait is her absolute and obsessive devotion to Naofumi, whom she sees as her savior. She perceives his soul as kind and pure, beyond his cynical exterior, and treats his orders as absolute doctrine. This devotion manifests as a fierce protectiveness; she desires nothing more than to be his shield in battle, seeing how he often disregards his own well-being to protect his companions. In her personal life, she is relentlessly affectionate, often trying to sneak into his room at night, leading to humorous skirmishes with other party members appointed to guard against her.

Her primary motivation stems from a deep-seated gratitude and love for Naofumi, who gave her a second chance at life. Having spent her early existence helpless and dependent, she refuses to be a burden and is driven to become a powerful asset to him. She seeks to gain his attention and affection by any means necessary, punishing anyone she perceives as getting in her way, including her own brother. Her goal is to ease Naofumi's burden, transforming from someone who always needed protection into someone who can provide it.

Within the story, Atla quickly becomes a valuable and formidable ally in the fight against the Waves of Calamity and contributes to the reconstruction of Raphtalia's village, Lurolona. Her role is that of an elite warrior and self-appointed bodyguard to Naofumi, acting as the shield for the Shield Hero. During the battle against the Phoenix, she fights on the front lines, using her abilities to shield the troops and land critical hits on the enemy. Her story culminates in a tragic sacrifice; when the Phoenix dies and creates a deadly explosion, Atla gives her own life to protect Naofumi and the rest of the army. In her final moments, she requests that her spirit be absorbed by the Legendary Shield, a wish that is granted, allowing her to remain with Naofumi forever as a permanent protective spirit that enhances his defense.

Atla's key relationships are intense and drive much of her development. Her connection with her brother, Fohl, is complex. While she loves him deeply, she constantly has to remind him that she is no longer the sickly child he obsesses over protecting. She often outwits him, whether by spiking his drink with sleeping pills or using pressure points to knock him unconscious, and acts as the true leader of the siblings. With Raphtalia, Atla is a direct love rival for Naofumi's attention. They clash constantly, with Raphtalia considering herself Naofumi's sword, while Atla fights to be his shield. Her relationship with the king, Aultcray Melromarc XXXII, is notable because Atla is revealed to be his niece; the king's long-perished sister married a Hakuko, making Atla and Fohl his grandniece and grandnephew. This revelation explains the king's uncharacteristic kindness towards her. Atla's development is dramatic, transforming from a bedridden, dying slave into a serene and powerful warrior with a genius-level battle sense. Her sacrifice profoundly changes Naofumi, forcing him to confront his own feelings and the weight of his mission.

Despite her blindness, Atla possesses extraordinary abilities. Her handicap grants her a form of spiritual clairvoyance, allowing her to see the flow of qi or life energy in everything around her, which makes her a supremely skilled combatant. She is a genius of the Hengen Musou style, able to perceive and strike enemy pressure points with surgical precision. Her combat style is fundamentally defensive, focused on dodging and waiting for an opening before counterattacking. She can learn and mimic defensive techniques based on Naofumi's own abilities. Her notable techniques include Collect, which allows her to redirect magical attacks, and Wall, which manifests an invisible barrier to obstruct enemies. Her ultimate technique is Musou Kassei, a state of heightened power that she can maintain for up to one minute using internal chi, or five minutes while actively drawing chi from her surroundings.