Millerna Sara Aston is the third princess of the kingdom of Asturia and a supporting character in the anime The Vision of Escaflowne. She is the daughter of King Grava Aston and Queen Therese Aston, and the youngest of three sisters. Her eldest sister, Marlene, is deceased and had married the Duke of Freid, while her second sister, Eries, holds a strong presence in the royal court and exerts considerable influence over Millerna's life.

As a princess, Millerna is expected to follow court protocol and accept a politically arranged marriage. She is betrothed to Dryden Fassa, the son of a wealthy merchant, a match arranged through family connections. Much of her personal conflict stems from the tension between her royal obligations and her own desires. She is not fully comfortable with life as royalty, and she harbors an ambition that sets her apart from the typical expectations of her station: she once wished to become a doctor and actively pursued medical studies. However, she was forced to abandon that path by her second sister, Eries, a decision that becomes a source of frustration and resentment. Millerna is described as accomplished and talented, with a strong-willed and spirited nature, and she does not shy away from hard work or effort when something matters to her.

In terms of personality, Millerna is emotional, outspoken, and sometimes impulsive, acting on her feelings rather than carefully weighing consequences. She is also caring and genuinely concerned for the well-being of others, which is part of what drew her to medicine in the first place. Her dissatisfaction with her arranged engagement and her longing for a more personally meaningful life lead her to take drastic action: she runs away from Asturia and joins the traveling group accompanying Van and his companions, hoping to escape her obligations and pursue her own dreams. This decision places her in the middle of the larger events of the story as part of the group's travels.

Her central motivation is the desire to live a life she has chosen for herself rather than one dictated by her position as a princess. This includes her wish for romantic love, which she initially believes she has found in Allen Schezar, a knight of Asturia's elite Knights Caeli. Her feelings for Allen are intense and largely one-sided, and this unrequited love shapes much of her early behavior. Her relationship with her betrothed, Dryden Fassa, is another central thread. She initially resents him as a symbol of the fate she is trying to escape, but over time she comes to recognize his genuine kindness, generosity, and sincere care for her. In the end, the two part on good terms, with Dryden telling her he will return when he feels more deserving of her. This parting becomes a turning point for Millerna, as she comes to understand that she cannot rely on others to make her happy and must take responsibility for her own life.

Her relationship with her sister Eries is defined by the clash between duty and personal ambition, since Eries imposed the restriction that ended Millerna's medical studies. Her friendship with the protagonist Hitomi provides her with an understanding companion, and her connection to the Aston family history, particularly through her deceased sister Marlene and Marlene's child, links her to the deeper personal entanglements of the story. Through her experiences away from the palace, Millerna develops from a rebellious young woman fleeing her problems into a more mature person who accepts her position and her responsibilities on her own terms.

Millerna has no combat abilities and does not pilot any of the war machines in the series. Her notable abilities come from her education and training: she possesses medical knowledge from her abandoned studies, along with the poise and social understanding expected of a princess. Her most defining personal traits are her determination, her compassion, and her willingness to act boldly, even when doing so means defying the expectations placed upon her.