Byleth

Byleth has a very subdued personality and has difficulty expressing emotion. As a newborn, they neither laughed nor cried. People they are especially close to, such as Jeralt, are able to tell how they are feeling despite their limited emotional visualizations. A handful of their students note that their outward expressions hardly vary. Some express surprise whenever they simply smile. According to Alois, this trait of theirs may in part come from their father, as Jeralt does not show his emotions easily either and can also be difficult to read. This should not be taken as them not having emotions at all however. Due to Jeralt concealing a lot of information about the world, they are showing curiosity about the influence of the Church of Seiros, the mystery of the Crests, and the dynamic of the three nations once they were immersed into the Officers Academy.

Their stoic demeanor slowly changes over the course of the Academy phase. Jeralt notices that they have become more expressive over the first few months, likely a result of the bonds they have formed with their students. Despite appearing indifferent, Byleth can be perceptive of their students and fellow faculty's problems and serve as a great source of emotional support for their issues. As the story unfolds, their emotions become more dynamic including feeling true sorrow, vengeance, and happiness.

Byleth has somewhat of a reputation from their earlier years in their father's mercenary corps for being a fearless and skilled warrior, gaining the moniker of 'Ashen Demon', due to the way they seemingly emotionlessly cut down their enemies in combat. This seems to be somewhat of a sensitive subject for them, as they seem uncomfortable when Hanneman mentions their moniker in their C support. It is notable that in the beginning of the game, Sothis refuses to let the player progress if they do not acknowledge their humanity to her, refuting the "Ghost" and "Demon" options, so it can be inferred that Byleth is uncomfortable with the Ashen Demon moniker because it dehumanizes them.