Warning- this article contains major spoilers. Proceed with caution.
Andromeda is a gold and black speckled bat, and is not described in any further detail in the books.
Andromeda seems to be a calm, forgiving, and kind bat, although her personality is not described.
In Prophecy of Bane, she accompanies Mareth and the other "Questers" on their journey to the Labyrinth. When Mareth suffers an injury, she flies him home alongside Howard. When she returns to Regalia, she and Howard are imprisonedfor not completing their mission. Thanks to Gregor, the two are freed after their trial.
In Curse of the Warmbloods, Andromeda suffers from the plague, and is hospitalized for the entire book before receiving the cure and eventually recovering.
Andromeda is named after the Greek figure, Andromeda, whose name roughly translates to "ruler of men."