Byleth

Byleth is a playable character and the main protagonist in Fire Emblem: Three Houses. As an Avatar, their name, gender, and birthday are customizable. Byleth serves as a professor at Garreg Mach Monastery. They possess the mysterious Crest of Flames and can wield the Sword of the Creator.

Personality
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 are uncomfortable with the Ashen Demon moniker because it dehumanises them.

The three house leaders of the Officer's Academy attest to their martial prowess and have deep, profound respect for their abilities and guidance in battle and in the classroom. They alongside the other students continue to refer to them as teacher or professor even after the time-skip. The three house leaders also deeply regret that Byleth had not chosen their house if they are not teaching theirs, seeing the value in their strengths.

According to Female Byleth in Heroes, she does not actually know the distinction between a friend and an ally due to their unusual childhood. Nevertheless, they are well liked by the student body, even those not in in their initial house. Their wisdom ultimately shapes the outcome of the events of Three Houses providing council and guidance to the house leader they chose to teach, helping to shape them into better warriors and leaders. Several students can develop an interest in them and eventually switch over to their house if not already in them. This results in them also joining their faction during the War, even those from enemy nations defecting to their cause, thus changing their fate. Those who do not join will often express regret facing them in battle. However, some students and faculty members, especially in the Crimson Flower route, will instead express outrage at them, vowing to kill them by their own hands.