He presents himself as a sharp-tongued lone wolf. Felix is so devoted to honing his swordsmanship that he will immediately challenge any strong person he meets to a duel. He values hard work and is disdainful of those that rely on their noble privilege or Crest.
Regarding Felix's supports, he seems to hate everything that is in any way connected to chivalry and good intentions and, therefore, has complicated relationships with the majority of Blue Lions. Felix calls Dedue a "boar's lapdog", straight up says Ingrid "go find a husband" instead of following her chivalrous ways and perceives Dimitri as a monster in human flesh pre-time skip. In his support with Sylvain, it is stated that Felix was much more easy-going during their childhoods and reminded Sylvain of a little brother. His bitterness and sudden change in character are due to the loss of his elder brother during the Tragedy of Duscur whose horrible and probably extremely painful death Ingrid and Rodrigue (Felix's father) view as a demise of a true knight.