Maybe? The twin of the traveler is shown to be working as a leading figure with the Abyss Order, who certainly seem to be one of the primary antagonists in Genshin Impact. That being said, it is highly likely that they are under some sort of spell or brainwashing effect to act evil.
In the game proper, the Abyss Order is shown to have a history of controlling people, using a combination of spells and manipulation to make Dvalin believe he was betrayed by Barbatos and Mondstadt, transforming him into Stormterror. It is possible, in fact likely, that the twin is under a similar effect. The introduction video whenever you begin a new Battle Pass month is a fairy tale about two royal children, a prince and a princess, who were sent on a quest by their father. One of the twin royals had fallen to some sort of evil along the way and become corrupted by illusions, now believing themselves to be the Queen of Darkness while the other royal twin must now begin their own journey. This would parallel with the male traveler being the main character, having their twin become known as “The Princess” and lead a group of dark wizards known for using illusions to brainwash victims into allies.