Concerning her visual appearance, it can be assumed she was also fairly good-looking like her two sisters, Narcissa and Bellatrix and her cousins, Sirius and Regulus. As for what side she is on...that's unknown. Despite being open-minded to the muggle culture, does not necessarily mean she's sided up with Dumbledore like her deceased cousin, Sirius Black. She certainly isn't with Voldemort, like her sisters and deceased cousin, Regulus Black. Probably on her own side with her own agenda for the upcoming wizard war.
Not much else is known about this third sister and there is little hint to her personality, although Nymphadora mentioned in OotP her mother's talent with household spells suggesting Andromeda may have the trait of neatness and organization.