In Fire Emblem: Three Houses, the Blue Lions house leader is Dimitri. His full name is Dimitri Alexandre Blaiddyd, and he is the crown prince of the Holy Kingdom of Faerghus.
Dimitri leads the Blue Lions at Garreg Mach Monastery, one of three student houses that Byleth can teach. The other house leaders are Edelgard von Hresvelg of the Black Eagles and Claude von Riegan of the Golden Deer. Choosing the Blue Lions sets the story on the route later associated with Azure Moon.
Dimitri’s background is central to the game’s plot. He survived the Tragedy of Duscur, an event that killed much of his family, and he later became closely connected with Dedue Molinaro, whom he rescued. His character changes dramatically after the five-year time skip, as trauma and revenge temporarily overwhelm his earlier image as a courteous, duty-bound prince.
A common mix-up is confusing Dimitri with Claude, since both are house leaders and prominent playable characters. Edelgard is also a house leader, but she commands the Black Eagles rather than the Blue Lions.