Felix Argyle (フェリックス・アーガイル), commonly known as Ferris (フェリス), is Crusch Karsten's knight.
Ferris has flax colored hair and yellow eyes. His casual wear consists of a blue collar with a ribbon, a blue dress, black tights, blue leggings, blue shoes, white arm covers, and a blue scarf tied around his right arm.
Ferris' cat like appendages are a result of his ancestors having Demi Human blood and because of this he was mistreated by his parents. The white and blue ribbons that he wears in his hair are a gift from Crusch and he always wears them no matter what he's wearing.
Ferris usually refers to himself in third person, calling himself Ferri-chan (フェリちゃん), though he will refer to himself as Ferris, Felix, or with female pronouns depending on the situation. He is absolutely loyal to Crusch and will do anything she says, however he will oppose her if needed. He also enjoys teasing other people, especially those that are surprised that he's male, using cutesy language and gestures to agitate them.
Although he hides it through his actions and words, Ferris has a severe view on death and values due to his job as a healer, leading him to hold those who no longer have any will to live in contempt from the bottom of his heart. He also hates weaklings because of his own complex, causing him to hate Subaru for a while, however he lets go of his feelings against him after seeing his actions during the battle against the Hakugei.
Ferris can get emotional when it comes to Crusch, such as the time when she was infected by Capella's dragon blood, becoming visibly stressed by her poor condition and even going as far as to demand to see a captured Sirius so he could torture her to gain information on how to cure her. He is constantly thankful for Crusch and everything that she's done for him in her life.
From the moment Ferris was born, he had cat like appendages due to his ancestors having Demi Human blood, causing his parents to mistreat him as they suspected an affair. He was locked in the basement of the Argyle Mansion for the first nine years of his life and was barely cared for. When Crusch found him, she described him as "a fragile boy who was painfully thin, was black from being covered in dirt and filth, and couldn't talk properly". He was subsequently taken to the Karsten Mansion to be cared for and his parents lost their standing with the Karsten Family, leading to their decline.
Water Magic: Ferris is considered to be the best Water Magic user on the continent, earning him the alias of Blue (青 Ao). He is also the youngest to be given this alias. By directing mana to anyone whose Od he's interfered with, he can make their body go out of control, enabling him to do things such as using their body like a puppet or destroying their circulatory system from inside. He can also die and heal himself to perfect condition, as shown during the battle against Petelguese, however it is unknown how many times he can do this.
- Immortal King's Sacrament (不死王の秘蹟 Fushiou no Hiseki): Ferris learned this magic from Biehn Argyle.
Divine Protection (加護 Kago): Ferris has the Divine Protection of Water (水の加護 Mizu no Kago), which makes him highly skilled at Water Magic.
Equipment: Ferris owns a short sword he received from Crusch, but as he doesn't like to damage his skin, he generally refuses to wield it. He has wielded it once in an effort to intimidate an unimpressed Capella.
- According to the author, Ferris' birthday is January 16.