Felix Argyle, commonly known as Ferris or Ferri-chan, is a demi-human possessing cat-like ears and a tail, traits stemming from dormant ancestry. Born into the noble House Argyle, he endured brutal abuse and neglect from his human parents, who confined him to a basement for nine years due to infidelity suspicions and shame over his heritage. Discovered malnourished, filthy, and nearly mute during an investigation by Crusch Karsten, he was rescued and brought to the Karsten mansion as her attendant. His parents subsequently lost standing with the Karsten family, leading to their decline. Following his rescue, Felix developed intense loyalty to Crusch, viewing her as his savior. Half a year later, they forged a pact: Crusch, seeking to inherit her father's ducal title under a masculine presentation, adopted male attire and mannerisms, while Felix embraced femininity in her stead. During this period, Prince Fourier Lugunica, Crusch's childhood friend, misheard "Felix" as "Ferris," cementing that name. Fourier later facilitated Felix's entry into the Royal Guard, where he befriended knights Reinhard van Astrea and Julius Juukulius. Felix typically refers to himself in third person using feminine pronouns or "Ferri-chan," employs cutesy speech patterns, and enjoys teasing those surprised by his biological gender. As a healer, he holds strong contempt for those lacking the will to live or exhibiting weakness, initially extending this disdain to Subaru Natsuki before respecting his resolve during the White Whale subjugation. His devotion manifests as extreme protectiveness, demanding to torture the Sin Archbishop Sirius for information when Crusch was infected. A pivotal trauma occurred when Crusch investigated Argyle mansion rumors alone and was captured by Felix's father, Biehn Argyle. Biehn, having murdered his wife in rage, forced Felix to use the forbidden "Immortal King's Sacrament" to resurrect her. Felix complied, but the incomplete spell briefly reanimated her corpse; she strangled Biehn before disintegrating. During this event, Felix learned the maid Hannah Regret was his biological mother, though she perished in the mansion fire with Biehn. Crusch freed herself before Felix could reach her. Felix possesses the Divine Protection of Water, granting exceptional aptitude for water magic focused on healing and life manipulation. Recognized as the kingdom's greatest healer with the title "Blue," he accelerates cellular regeneration to reattach limbs, cure near-fatal injuries, or disrupt biological functions like neural pain signals. His pseudo-immortality allows rapid regeneration from non-fatal wounds and self-resurrection via mana expenditure, weakening his combat capabilities. Enhanced demi-human senses detect subtle environmental shifts. Though owning a short sword gifted by Crusch, he avoids physical combat to protect his skin and lacks proficiency. The Immortal King's Sacrament remains dangerously flawed, producing unstable reanimations. Following Fourier's death from a royal illness—which Felix could not cure—and Crusch's selection as a royal candidate, Felix was knighted as her sworn guardian. He supported her political goal to sever Lugunica's dragon covenant and reform the government. After Crusch lost her memories to Gluttony's authority, Felix became her primary caretaker, demonstrating unwavering commitment. His relationship with Subaru evolved into mutual respect, though he expressed sorrow when forced to kill him during Petelgeuse's possession.