Byleth is the protagonist of the video game Fire Emblem Three Houses. Their sex is determined by the player, but both sexes have dark teal hair, blue eyes, and light skin. They share their body with Sothis, a god. They can fuse with Sothis to gain additional powers, whereupon their hair color changes to a neon green. Byleth can fight with any weapon, including magic, but is most commonly depicted wielding their personal weapon, the Sword of the Creator. (Said sword is actually forged from the bones of Sothis.) In the story of Three Houses, Byleth serves as a professor for the Officers' Academy.

Byleth is very stoic and has difficulty expressing their emotions, so they are often depicted with a blank or emotionless expression.

Byleth is functionally human for the purposes of species tagging.

This character has alternate forms: Byleth (male) and Byleth (female).