Ferris has flax-colored hair and yellow eyes, along with distinct cat ears and a tail that highlight his demi-human heritage. His usual outfit consists of a blue collar with a ribbon, a primarily white dress featuring a light blue chest and hem with thin white stripes, and matching light blue ribbons tied at the waist. A large white ribbon is tied at the back of the dress, complemented by black tights, blue leggings with white stripes, dark blue shoes, and detached white arm covers that leave his shoulders bare. He also wears a dark blue scarf around his right arm, a dark blue ribbon around his left ankle, and white-and-blue ribbon hair accessories on both sides of his head, which he keeps on regardless of attire.

On official occasions or during major military operations such as the Subjugation of the White Whale and Sloth, Ferris wears the standard uniform of the Royal Army. In Arc 5, he is also seen wearing a yukata, showcasing a more casual and traditional look.

Ferris usually refers to himself in the third person as Ferri-chan (フェリちゃん), though he will use Ferris or Felix depending on the situation. Playful and teasing by nature, he enjoys flustering others, especially those surprised to learn he is male, by using cutesy speech and gestures. Despite his cheerful demeanor, Ferris holds deep convictions about life and death. As a healer, he despises those who abandon their will to live and hides his frustration behind playful words. Due to his own trauma and weakness growing up, he harbors resentment toward those he deems helpless, a trait that initially caused tension between him and Subaru. He also struggles with guilt and disgust whenever he fails to save a life, such as when faced with the victims of the Sin Archbishop of Lust. His resentment toward his father’s cruelty stems from being physically weak and abused, which left him feeling unable to protect Crusch.

In his youth, Ferris endured years of abuse and confinement from his parents, and upon being freed, he tried to attack his mother out of anger, only to be stopped by Crusch. After she rescued, cleaned, and cared for him, Ferris became deeply loyal to her. Wanting to be of use, he trained in swordsmanship and later mastered healing magic. When Crusch confided in him about her struggles with expectations and identity, the two made a promise that Ferris would act as her feminine side while she embodied the strength he lacked. This promise became the foundation of their bond, with Ferris even adopting the name Ferris after a misunderstanding. His devotion runs so deep that he often makes decisions out of emotion, such as trying to sever alliances or torture enemies to save her, showing the depth of his dependence and care for the people he loves.

Divine Protection (加護, Kago): Ferris possesses the Divine Protection of Water (水の加護, Mizu no Kago), which grants him exceptional affinity and mastery over water magic.

Magic (魔法, Mahō): Ferris’ natural talent and bloodline, combined with his Divine Protection, make him one of the most gifted water mages in the kingdom.

Healing Magic (治癒魔法, Chiyu Mahō): Ferris can channel Water Mana through a person’s body to accelerate their natural healing process and restore physical health. His healing can instantly cure wounds and ailments, though it cannot treat mental afflictions. Renowned as the greatest healer in the kingdom—possibly the world—Ferris earned the title “Blue” for his unparalleled skill. He can revive individuals on the brink of death and heal even the most severe injuries without leaving scars. His mastery extends to complex cases, such as reattaching Crusch’s severed arm after it was cut off by the Sin Archbishop of Greed. However, Ferris is unable to cure the illness that wiped out the royal family or the dragon’s blood curse inflicted by the Sin Archbishop of Lust. Additionally, his healing requires that the target still have a trace of life remaining; once dead, a person cannot be revived by his magic.

Bodily Function Manipulation: Ferris can manipulate internal bodily functions using water magic. He can disrupt a person’s body, control their movements like a puppet, or destroy the circulatory system of their Gate from within. He is also capable of suppressing pain, though he avoids doing so to prevent his patients from overexerting themselves.

Sacrament of the Immortal King (不死王の秘蹟, Fushiou no Hiseki): This ancient spell, created by an unknown Witch, allows the reanimation of the dead. The full version of the spell has been lost, and only fragments remain that can produce soulless, decaying corpse soldiers. Ferris once attempted this spell on his mother’s corpse, resulting in a brief moment of life during which she killed his abusive father before turning to dust. Her final smile before vanishing remains a mystery, suggesting that something beyond simple reanimation occurred.

Pseudo-Immortality: Ferris’ deep connection with Water Mana grants him rapid cellular regeneration, allowing him to resurrect after death. However, this ability weakens his body over time, and the limits of this power remain unknown.

Heightened Senses: Due to his demi-human blood, Ferris’ senses are sharper than those of ordinary humans. His cat-like ears can detect subtle changes in his surroundings, even sensing when someone looks at him. His hearing is so keen that only Reinhard van Astrea has managed to approach him undetected.

Felix Argyle was born with cat-like features, a trait from his father’s bloodline that caused his parents to mistreat and imprison him for nine years. During this time, a young Crusch Karsten visited the Argyle household and discovered the abused boy. Overcome by hatred, Felix attempted to attack his mother but instead wounded Crusch. His latent healing powers awakened in that moment, saving her life. Deeply moved by her kindness and words, Felix swore loyalty to Crusch and became her attendant, taking refuge in the Karsten household. In time, he formed an unbreakable bond with her, promising to act as her feminine side while she pursued her dream of becoming a duke. Under her care, Felix, now known as Ferris, found a new purpose, developing his skills in healing magic and establishing lifelong friendships with Crusch and Fourier Lugunica.

As Ferris grew older, he joined the Royal Guard at age eighteen, supported by both Crusch and Fourier. Though initially alienated for his demi-human traits and preference for feminine attire, Ferris gained the respect of his peers through his unmatched healing abilities and valor. His loyalty to Crusch was tested when dark rumors about his family surfaced. Returning to the Argyle estate, Ferris was forced to confront his abusive father and revive his murdered mother using the forbidden Sacrament of the Immortal King. The spell only partially succeeded, and his undead mother killed his father before disintegrating, leaving Ferris emotionally scarred and with a lasting hatred for those who give up on life. Shortly after, Fourier succumbed to a mysterious illness, and though Ferris desperately tried to save him, he ultimately accepted Fourier’s wish to die with dignity.

Years later, Ferris continued to serve the kingdom faithfully, joining Julius and Reinhard in a diplomatic mission to the Vollachia Empire. When a coup erupted, the trio defended Emperor Vincent Vollachia, fighting against the Empire’s powerful Divine Generals. Ferris’ healing and courage helped secure their survival, allowing them to escape and end the conflict with a peace treaty between the two nations. His experiences on the battlefield and the personal losses he endured shaped Ferris into one of Lugunica’s most skilled healers and most loyal knights, forever devoted to Crusch Karsten and the ideals she inspired in him.